দ্বিপদী উপপাদ্য: সংশোধিত সংস্করণের মধ্যে পার্থক্য

উইকিপিডিয়া, মুক্ত বিশ্বকোষ থেকে
বিষয়বস্তু বিয়োগ হয়েছে বিষয়বস্তু যোগ হয়েছে
SushmitaSwarna (আলোচনা | অবদান)
"Binomial theorem" পাতাটি অনুবাদ করে তৈরি করা হয়েছে
SushmitaSwarna (আলোচনা | অবদান)
সম্পাদনা সারাংশ নেই
১ নং লাইন: ১ নং লাইন:
{{কাজ চলছে/২০১৯}}
{{Image frame|width=215|content=<math>
{{Image frame|width=215|content=<math>
\begin{array}{c}
\begin{array}{c}
১২ নং লাইন: ১২ নং লাইন:
\end{array}
\end{array}
</math>|caption=The [[binomial coefficient]] <math>\tbinom nb</math> appears as the ''b''th entry in the ''n''th row of [[Pascal's triangle]] (counting starts at {{val|0}}). Each entry is the sum of the two above it.}} প্রাথমিক বীজগণিতে, '''দ্বিপদী উপপাদ্য''' (বা '''দ্বিপদী বিস্তার''' ) একটি দ্বিপদী রাশির [[সূচকীকরণ|সূচকের]] বীজগাণিতিক সম্প্রসারণ বর্ণনা করে। এই উপপাদ্য অনুযায়ী, একটি {{math|(''x''&nbsp;+&nbsp;''y'')<sup>''n''</sup>}} আকারের বহুপদীকে কয়েকটি {{math|''a x''<sup>''b''</sup> ''y''<sup>''c''</sup>}} আকারের রাশির [[ সঙ্কলন |সমষ্টি]] রূপে প্রকাশ করা সম্ভব, যেখানে {{Mvar|b}} এবং {{Mvar|c}} সূচকদ্বয় প্রত্যেকে অঋণাত্মক [[স্বাভাবিক সংখ্যা|পূর্ণসংখ্যা]] ও {{math|''b'' + ''c'' {{=}} ''n''}}, এবং প্রতিটি রাশির [[সহগ]] {{Mvar|a}} একটি নির্দিষ্ট [[স্বাভাবিক সংখ্যা|ধনাত্মক পূর্ণসংখ্যা]] যার মান {{Mvar|n}} ও {{Mvar|b}} এর উপর নির্ভর করে। উদাহরণস্বরূপ, {{Mvar|n}} = 4 এর জন্য-
</math>|caption=The [[binomial coefficient]] <math>\tbinom nb</math> appears as the ''b''th entry in the ''n''th row of [[Pascal's triangle]] (counting starts at {{val|0}}). Each entry is the sum of the two above it.}} প্রাথমিক বীজগণিতে, '''দ্বিপদী উপপাদ্য''' (বা '''দ্বিপদী বিস্তার''' ) একটি দ্বিপদী রাশির [[সূচকীকরণ|সূচকের]] বীজগাণিতিক সম্প্রসারণ বর্ণনা করে। এই উপপাদ্য অনুযায়ী, একটি {{math|(''x''&nbsp;+&nbsp;''y'')<sup>''n''</sup>}} আকারের বহুপদীকে কয়েকটি {{math|''a x''<sup>''b''</sup> ''y''<sup>''c''</sup>}} আকারের রাশির [[ সঙ্কলন |সমষ্টি]] রূপে প্রকাশ করা সম্ভব, যেখানে {{Mvar|b}} এবং {{Mvar|c}} সূচকদ্বয় প্রত্যেকে অঋণাত্মক [[স্বাভাবিক সংখ্যা|পূর্ণসংখ্যা]] ও {{math|''b'' + ''c'' {{=}} ''n''}}, এবং প্রতিটি রাশির [[সহগ]] {{Mvar|a}} একটি নির্দিষ্ট [[স্বাভাবিক সংখ্যা|ধনাত্মক পূর্ণসংখ্যা]] যার মান {{Mvar|n}} ও {{Mvar|b}} এর উপর নির্ভর করে। উদাহরণস্বরূপ, {{Mvar|n}} = 4 এর জন্য-

:<math>(x+y)^4 = x^4 + 4 x^3y + 6 x^2 y^2 + 4 x y^3 + y^4. </math>

The coefficient {{mvar|a}} in the term of {{math|''a x''<sup>''b''</sup> ''y''<sup>''c''</sup>}} is known as the [[binomial coefficient]] <math>\tbinom nb</math> or <math>\tbinom nc</math> (the two have the same value). These coefficients for varying {{mvar|n}} and {{mvar|b}} can be arranged to form [[Pascal's triangle]]. These numbers also arise in [[combinatorics]], where <math>\tbinom nb</math> gives the number of different [[combinations]] of {{mvar|b}} [[element (mathematics)|elements]] that can be chosen from an {{mvar|n}}-element [[set (mathematics)|set]]. Therefore <math>\tbinom nb</math> is often pronounced as "''n'' choose ''b''".

==History==
Special cases of the binomial theorem were known since at least the 4th century BC when [[Greek mathematics|Greek mathematician]] [[Euclid]] mentioned the special case of the binomial theorem for exponent&nbsp;2.<ref name=wolfram>{{cite web|url=http://mathworld.wolfram.com/BinomialTheorem.html|title=Binomial Theorem|website=Wolfram MathWorld|last=Weisstein|first=Eric W.}}</ref><ref name="Coolidge">{{cite journal|title=The Story of the Binomial Theorem|first=J. L.|last=Coolidge|journal=The American Mathematical Monthly|volume=56|issue=3|date=1949|pp=147–157|doi=10.2307/2305028|jstor = 2305028}}</ref> There is evidence that the binomial theorem for cubes was known by the 6th century AD in India.<ref name=wolfram /><ref name="Coolidge" />

Binomial coefficients, as combinatorial quantities expressing the number of ways of selecting ''k'' objects out of ''n'' without replacement, were of interest to ancient Indian mathematicians. The earliest known reference to this combinatorial problem is the ''Chandaḥśāstra'' by the Indian lyricist [[Pingala]] (c. 200 BC), which contains a method for its solution.<ref name=Chinese>{{cite book|title=A history of Chinese mathematics|author1=Jean-Claude Martzloff|author2=S.S. Wilson|author3=J. Gernet|author4=J. Dhombres|publisher=Springer|year=1987}}</ref>{{rp|230}} The commentator [[Halayudha]] from the 10th century AD explains this method using what is now known as [[Pascal's triangle]].<ref name=Chinese /> By the 6th century AD, the Indian mathematicians probably knew how to express this as a quotient <math>\frac{n!}{(n-k)!k!}</math>,<ref name="Biggs">{{cite journal|last=Biggs|first=N. L.|title=The roots of combinatorics|journal=Historia Math.|volume=6|date=1979|issue=2|pages=109–136|doi=10.1016/0315-0860(79)90074-0}}</ref> and a clear statement of this rule can be found in the 12th century text ''Lilavati'' by [[Bhāskara II|Bhaskara]].<ref name="Biggs" />

The first formulation of the binomial theorem and the table of binomial coefficients, to our knowledge, can be found in a work by [[Al-Karaji]], quoted by [[Al-Samaw'al]] in his "al-Bahir".<ref>{{Cite web|url=https://core.ac.uk/download/pdf/82000184.pdf|website=core.ac.uk|access-date=2019-01-08|title=THE BINOMIAL THEOREM : A WIDESPREAD CONCEPT IN MEDIEVAL ISLAMIC MATHEMATICS|page=401}}</ref><ref>{{Cite journal|last=|first=|date=|title=Taming the unknown. A history of algebra from antiquity to the early twentieth century|url=http://www.ams.org/journals/bull/2015-52-04/S0273-0979-2015-01491-6/S0273-0979-2015-01491-6.pdf|journal=Bulletin of the American Mathematical Society|volume=|pages=|via=|page=727|quote=However, algebra advanced in other respects. Around 1000, al-Karaji stated the binomial theorem}}</ref><ref>{{Cite book|url=https://books.google.com/?id=vSkClSvU_9AC&pg=PA62#v=onepage&q&f=true|title=The Development of Arabic Mathematics: Between Arithmetic and Algebra|last=Rashed|first=R.|date=1994-06-30|publisher=Springer Science & Business Media|isbn=9780792325659|language=en|page=63}}</ref> [[Al-Karaji]] described the triangular pattern of the binomial coefficients<ref name=Karaji>{{MacTutor|id=Al-Karaji|title=Abu Bekr ibn Muhammad ibn al-Husayn Al-Karaji}}</ref> and also provided a [[mathematical proof]] of both the binomial theorem and Pascal's triangle, using an early form of [[mathematical induction]].<ref name=Karaji /> The Persian poet and mathematician [[Omar Khayyam]] was probably familiar with the formula to higher orders, although many of his mathematical works are lost.<ref name="Coolidge" /> The binomial expansions of small degrees were known in the 13th century mathematical works of [[Yang Hui]]<ref>{{cite web
| last = Landau
| first = James A.
| title =Historia Matematica Mailing List Archive: Re: [HM] Pascal's Triangle
| work = Archives of Historia Matematica
| format = mailing list email
| accessdate = 2007-04-13
| date = 1999-05-08
| url = http://archives.math.utk.edu/hypermail/historia/may99/0073.html
}}</ref> and also [[Chu Shih-Chieh]].<ref name="Coolidge" /> Yang Hui attributes the method to a much earlier 11th century text of [[Jia Xian]], although those writings are now also lost.<ref name=Chinese />{{rp|142}}

In 1544, [[Michael Stifel]] introduced the term "binomial coefficient" and showed how to use them to express <math>(1+a)^n</math> in terms of <math>(1+a)^{n-1}</math>, via "Pascal's triangle".<ref name=Kline>{{cite book|title=History of mathematical thought|first=Morris|last=Kline|page=273|publisher=Oxford University Press|year=1972}}</ref> [[Blaise Pascal]] studied the eponymous triangle comprehensively in the [[treatise]] ''Traité du triangle arithmétique'' (1653). However, the pattern of numbers was already known to the European mathematicians of the late Renaissance, including Stifel, [[Niccolò Fontana Tartaglia]], and [[Simon Stevin]].<ref name=Kline />

[[Isaac Newton]] is generally credited with the generalized binomial theorem, valid for any rational exponent.<ref name=Kline /><ref>{{cite book|title=Elements of the History of Mathematics Paperback|date= 18 November 1998|first= N.|last=Bourbaki|others= J. Meldrum (Translator)|isbn=978-3-540-64767-6}}</ref>

== Theorem statement ==
According to the theorem, it is possible
to expand any power of {{math|''x''&nbsp;+&nbsp;''y''}} into a sum of the form
:<math>(x+y)^n = {n \choose 0}x^n y^0 + {n \choose 1}x^{n-1}y^1 + {n \choose 2}x^{n-2}y^2 + \cdots + {n \choose n-1}x^1 y^{n-1} + {n \choose n}x^0 y^n,
</math>
where each <math> \tbinom nk </math> is a specific positive integer known as a [[binomial coefficient]]. (When an exponent is zero, the corresponding power expression is taken to be 1 and this multiplicative factor is often omitted from the term. Hence one often sees the right side written as <math>\binom{n}{0} x^n + \ldots</math>.) This formula is also referred to as the '''binomial formula''' or the '''binomial identity'''. Using [[Capital-sigma notation|summation notation]], it can be written as
:<math>(x+y)^n = \sum_{k=0}^n {n \choose k}x^{n-k}y^k = \sum_{k=0}^n {n \choose k}x^{k}y^{n-k}.
</math>
The final expression follows from the previous one by the symmetry of ''x'' and ''y'' in the first expression, and by comparison it follows that the sequence of binomial coefficients in the formula is symmetrical.
A simple variant of the binomial formula is obtained by [[substitution (algebra)|substituting]] 1 for ''y'', so that it involves only a single [[Variable (mathematics)|variable]]. In this form, the formula reads
:<math>(1+x)^n = {n \choose 0}x^0 + {n \choose 1}x^1 + {n \choose 2}x^2 + \cdots + {n \choose {n-1}}x^{n-1} + {n \choose n}x^n,</math>
or equivalently
:<math>(1+x)^n = \sum_{k=0}^n {n \choose k}x^k.</math>

== Examples ==

The most basic example of the binomial theorem is the formula for the [[Square (algebra)|square]] of {{math|''x'' + ''y''}}:

:<math>(x + y)^2 = x^2 + 2xy + y^2.</math>

The binomial coefficients 1, 2, 1 appearing in this expansion correspond to the second row of Pascal's triangle. (Note that the top "1" of the triangle is considered to be row 0, by convention.) The coefficients of higher powers of {{math|''x'' + ''y''}} correspond to lower rows of the triangle:

:<math>
\begin{align}
(x+y)^3 & = x^3 + 3x^2y + 3xy^2 + y^3, \\[8pt]
(x+y)^4 & = x^4 + 4x^3y + 6x^2y^2 + 4xy^3 + y^4, \\[8pt]
(x+y)^5 & = x^5 + 5x^4y + 10x^3y^2 + 10x^2y^3 + 5xy^4 + y^5, \\[8pt]
(x+y)^6 & = x^6 + 6x^5y + 15x^4y^2 + 20x^3y^3 + 15x^2y^4 + 6xy^5 + y^6, \\[8pt]
(x+y)^7 & = x^7 + 7x^6y + 21x^5y^2 + 35x^4y^3 + 35x^3y^4 + 21x^2y^5 + 7xy^6 + y^7.
\end{align}
</math>
Several patterns can be observed from these examples. In general, for the expansion {{math|(''x'' + ''y'')<sup>''n''</sup>}}:
# the powers of {{mvar|x}} start at {{mvar|n}} and decrease by 1 in each term until they reach 0 (with {{math|1=''x''<sup>0</sup> = 1}}, often unwritten);
# the powers of {{mvar|y}} start at 0 and increase by 1 until they reach {{mvar|n}};
# the {{mvar|n}}th row of Pascal's Triangle will be the coefficients of the expanded binomial when the terms are arranged in this way;
# the number of terms in the expansion before like terms are combined is the sum of the coefficients and is equal to {{math|2<sup>''n''</sup>}}; and
# there will be {{math|''n'' + 1}} terms in the expression after combining like terms in the expansion.

The binomial theorem can be applied to the powers of any binomial. For example,

:<math>\begin{align}
(x+2)^3 &= x^3 + 3x^2(2) + 3x(2)^2 + 2^3 \\
&= x^3 + 6x^2 + 12x + 8.\end{align}</math>

For a binomial involving subtraction, the theorem can be applied by using the form {{math|1=(''x'' − ''y'')<sup>''n''</sup> = (''x'' + (−''y''))<sup>''n''</sup>}}. This has the effect of changing the sign of every other term in the expansion:
:<math>(x-y)^3 = (x+(-y))^3 = x^3 + 3x^2(-y) + 3x(-y)^2 + (-y)^3 = x^3 - 3x^2y + 3xy^2 -y^3.</math>

=== Geometric explanation ===
[[File:binomial_theorem_visualisation.svg|thumb|300px|Visualisation of binomial expansion up to the 4th power]]
For positive values of ''a'' and ''b'', the binomial theorem with {{math|1=''n''&nbsp;=&nbsp;2}} is the geometrically evident fact that a square of side {{math|''a'' + ''b''}} can be cut into a square of side ''a'', a square of side ''b'', and two rectangles with sides ''a'' and ''b''. With {{math|1=''n''&nbsp;=&nbsp;3}}, the theorem states that a cube of side {{math|''a'' + ''b''}} can be cut into a cube of side ''a'', a cube of side ''b'', three ''a''×''a''×''b'' rectangular boxes, and three ''a''×''b''×''b'' rectangular boxes.

In [[calculus]], this picture also gives a geometric proof of the [[derivative]] <math>(x^n)'=nx^{n-1}:</math><ref name="barth2004">{{cite journal | last = Barth | first = Nils R.| title = Computing Cavalieri's Quadrature Formula by a Symmetry of the ''n''-Cube | doi = 10.2307/4145193 | jstor = 4145193 | journal = The American Mathematical Monthly| issn = 0002-9890| volume = 111| issue = 9| pages = 811–813 | date=2004 | pmid = | pmc =| postscript = , [http://nbarth.net/math/papers/barth-01-cavalieri.pdf author's copy], [http://nbarth.net/math/papers/ further remarks and resources]}}</ref> if one sets <math>a=x</math> and <math>b=\Delta x,</math> interpreting ''b'' as an [[infinitesimal]] change in ''a,'' then this picture shows the infinitesimal change in the volume of an ''n''-dimensional [[hypercube]], <math>(x+\Delta x)^n,</math> where the coefficient of the linear term (in <math>\Delta x</math>) is <math>nx^{n-1},</math> the area of the ''n'' faces, each of dimension <math>(n-1):</math>
:<math>(x+\Delta x)^n = x^n + nx^{n-1}\Delta x + \binom{n}{2}x^{n-2}(\Delta x)^2 + \cdots.</math>
Substituting this into the [[definition of the derivative]] via a [[difference quotient]] and taking limits means that the higher order terms, <math>(\Delta x)^2</math> and higher, become negligible, and yields the formula <math>(x^n)'=nx^{n-1},</math> interpreted as
:"the infinitesimal rate of change in volume of an ''n''-cube as side length varies is the area of ''n'' of its <math>(n-1)</math>-dimensional faces".
If one integrates this picture, which corresponds to applying the [[fundamental theorem of calculus]], one obtains [[Cavalieri's quadrature formula]], the integral <math>\textstyle{\int x^{n-1}\,dx = \tfrac{1}{n} x^n}</math> – see [[Cavalieri's quadrature formula#Proof|proof of Cavalieri's quadrature formula]] for details.<ref name="barth2004" />

{{clear}}

== Binomial coefficients ==
{{Main|Binomial coefficient}}
The coefficients that appear in the binomial expansion are called '''binomial coefficients'''. These are usually written <math> \tbinom nk </math>, and pronounced “''n'' choose ''k''”.

=== Formulae ===
The coefficient of ''x''<sup>''n''−''k''</sup>''y''<sup>''k''</sup> is given by the formula

:<math>{n \choose k} = \frac{n!}{k! (n-k)!}</math>

which is defined in terms of the [[factorial]] function ''n''!. Equivalently, this formula can be written

:<math>{n \choose k} = \frac{n (n-1) \cdots (n-k+1)}{k (k-1) \cdots 1} = \prod_{\ell=1}^k \frac{n-\ell+1}{\ell} = \prod_{\ell=0}^{k-1} \frac{n-\ell}{k - \ell}</math>

with ''k'' factors in both the numerator and denominator of the [[Fraction (mathematics)|fraction]]. Note that, although this formula involves a fraction, the binomial coefficient <math> \tbinom nk </math> is actually an [[integer]].

=== Combinatorial interpretation ===
The binomial coefficient <math> \tbinom nk </math> can be interpreted as the number of ways to choose ''k'' elements from an ''n''-element set. This is related to binomials for the following reason: if we write {{math|1=(''x''&nbsp;+&nbsp;''y'')<sup>''n''</sup>}} as a [[Product (mathematics)|product]]
:<math>(x+y)(x+y)(x+y)\cdots(x+y),</math>
then, according to the [[distributive law]], there will be one term in the expansion for each choice of either ''x'' or ''y'' from each of the binomials of the product. For example, there will only be one term ''x''<sup>''n''</sup>, corresponding to choosing ''x'' from each binomial. However, there will be several terms of the form ''x''<sup>''n''−2</sup>''y''<sup>2</sup>, one for each way of choosing exactly two binomials to contribute a ''y''. Therefore, after [[combining like terms]], the coefficient of ''x''<sup>''n''−2</sup>''y''<sup>2</sup> will be equal to the number of ways to choose exactly 2 elements from an ''n''-element set.

== Proofs ==

=== Combinatorial proof ===

==== Example ====
The coefficient of ''xy''<sup>2</sup> in

:<math>\begin{align}
(x+y)^3 &= (x+y)(x+y)(x+y) \\
&= xxx + xxy + xyx + \underline{xyy} + yxx + \underline{yxy} + \underline{yyx} + yyy \\
&= x^3 + 3x^2y + \underline{3xy^2} + y^3.
\end{align}</math>

equals <math>\tbinom{3}{2}=3</math> because there are three ''x'',''y'' strings of length 3 with exactly two ''y'''s, namely,

:<math>xyy, \; yxy, \; yyx,</math>

corresponding to the three 2-element subsets of {&nbsp;1,&nbsp;2,&nbsp;3&nbsp;}, namely,

:<math>\{2,3\},\;\{1,3\},\;\{1,2\}, </math>

where each subset specifies the positions of the ''y'' in a corresponding string.

==== General case ====
Expanding {{math|1=(''x''&nbsp;+&nbsp;''y'')<sup>''n''</sup>}} yields the sum of the 2<sup>&nbsp;''n''</sup> products of the form {{math|1=''e''<sub>1</sub>''e''<sub>2</sub>&nbsp;...&nbsp;''e''<sub>&nbsp;''n''</sub>}} where each ''e''<sub>&nbsp;''i''</sub> is ''x'' or&nbsp;''y''. Rearranging factors shows that each product equals ''x''<sup>''n''−''k''</sup>''y''<sup>''k''</sup> for some ''k'' between 0 and&nbsp;''n''. For a given ''k'', the following are proved equal in succession:
* the number of copies of {{math|1=''x''<sup>''n''&nbsp;−&nbsp;''k''</sup>''y''<sup>''k''</sup>}} in the expansion
* the number of ''n''-character ''x'',''y'' strings having ''y'' in exactly ''k'' positions
* the number of ''k''-element subsets of {{math|1={{mset|&nbsp;1,&nbsp;2,&nbsp;...,&nbsp;''n''}}}}
* <math>{n \choose k}</math> (this is either by definition, or by a short combinatorial argument if one is defining <math>{n \choose k}</math> as <math>\frac{n!}{k! (n-k)!}</math>).
This proves the binomial theorem.

=== Inductive proof ===
[[mathematical induction|Induction]] yields another proof of the binomial theorem. When {{math|1=''n''&nbsp;=&nbsp;0}}, both sides equal 1, since {{math|1=''x''<sup>0</sup>&nbsp;=&nbsp;1}} and <math>\tbinom{0}{0}=1</math>.
Now suppose that the equality holds for a given ''n''; we will prove it for {{math|1=''n''&nbsp;+&nbsp;1}}.
For {{math|1=''j'',&nbsp;''k''&nbsp;≥&nbsp;0}}, let {{math|1=[''ƒ''(''x'',&nbsp;''y'')]<sub>&nbsp;''j,k''</sub>}} denote the coefficient of {{math|1=''x''<sup>''j''</sup>''y''<sup>''k''</sup>}} in the polynomial {{math|1=''ƒ''(''x'',&nbsp;''y'')}}.
By the inductive hypothesis, {{math|1=(''x''&nbsp;+&nbsp;''y'')<sup>''n''</sup>}} is a polynomial in ''x'' and ''y'' such that {{math|1=[(''x''&nbsp;+&nbsp;''y'')<sup>''n''</sup>]<sub>&nbsp;''j,k''</sub>}} is <math>\tbinom{n}{k}</math> if {{math|1=''j''&nbsp;+&nbsp;''k''&nbsp;=&nbsp;''n''}}, and 0 otherwise.
The identity

:<math> (x+y)^{n+1} = x(x+y)^n + y(x+y)^n</math>

shows that {{math|1=(''x''&nbsp;+&nbsp;''y'')<sup>''n'' + 1</sup>}} also is a polynomial in ''x'' and ''y'', and

:<math> [(x+y)^{n+1}]_{j,k} = [(x+y)^n]_{j-1,k} + [(x+y)^n]_{j,k-1},</math>

since if {{math|1=''j''&nbsp;+&nbsp;''k''&nbsp;=&nbsp;''n''&nbsp;+&nbsp;1}}, then {{math|1=(''j''&nbsp;−&nbsp;1)&nbsp;+&nbsp;''k''&nbsp;=&nbsp;''n''}} and {{math|1=''j''&nbsp;+&nbsp;(''k''&nbsp;−&nbsp;1)&nbsp;=&nbsp;''n''}}. Now, the right hand side is

:<math> \binom{n}{k} + \binom{n}{k-1} = \binom{n+1}{k},</math>

by [[Pascal's identity]].<ref>[http://proofs.wiki/Binomial_theorem Binomial theorem] – inductive proofs {{webarchive |url=https://web.archive.org/web/20150224130932/http://proofs.wiki/Binomial_theorem |date=February 24, 2015 }}</ref> On the other hand, if {{math|1=''j''&nbsp;+''k''&nbsp;≠&nbsp;''n''&nbsp;+&nbsp;1}}, then {{math|1=(''j''&nbsp;–&nbsp;1)&nbsp;+&nbsp;''k''&nbsp;≠&nbsp;''n''}} and {{math|1=''j''&nbsp;+(''k''&nbsp;–&nbsp;1)&nbsp;≠&nbsp;''n''}}, so we get {{math|1=0&nbsp;+&nbsp;0&nbsp;=&nbsp;0}}. Thus

:<math>(x+y)^{n+1} = \sum_{k=0}^{n+1} \binom{n+1}{k} x^{n+1-k} y^k,</math>

which is the inductive hypothesis with {{math|1=''n''&nbsp;+&nbsp;1}} substituted for ''n'' and so completes the inductive step.

== Generalizations ==

=== Newton's generalized binomial theorem ===
{{Main|Binomial series}}
Around 1665, [[Isaac Newton]] generalized the binomial theorem to allow real exponents other than nonnegative integers. (The same generalization also applies to [[complex number|complex]] exponents.) In this generalization, the finite sum is replaced by an [[infinite series]]. In order to do this, one needs to give meaning to binomial coefficients with an arbitrary upper index, which cannot be done using the usual formula with factorials. However, for an arbitrary number ''r'', one can define

:<math>{r \choose k}=\frac{r(r-1) \cdots (r-k+1)}{k!} =\frac{(r)_k}{k!},</math>
<!--
This is not the same as \frac{r!}{k!(r−k)!}. Factorials are typically only defined on natural number arguments, but even if you are using factorials generalized (e.g. by the \Gamma function) to non-integer values, they are still undefined on the negative integers. To get the usual binomial theorem as a special case of this so-called generalization, we had better define the binomial coefficient when ''r'' is an integer, but in that case ''r''−''k'' will be a negative integer for sufficiently large ''k'', so one cannot use any formula involving the factorial <math>(r−k)!</math>.

This negative comment about "not the same as…" seems to be needed. People keep coming along and completing this formula with this expression involving factorials, missing the point of this section.
~~~~perhaps someone could put a better explanation in! Here is an attempt!.
The problem with substituting \frac{r!}{k!(r−k)!} is that the ! ends up being used for negative numbers which doesn't work with the definition of !. Consequently, the notation here is used because if you look at it for a negative value of n, the value is still defined with this notation. That being said, many text books are careless about it.
-->
where <math>(\cdot)_k</math> is the [[Pochhammer symbol]], here standing for a [[falling factorial]]. This agrees with the usual definitions when ''r'' is a nonnegative integer. Then, if ''x'' and ''y'' are real numbers with <span class="texhtml">|''x''|&nbsp;>&nbsp;|''y''|</span>,<ref name=convergence group=Note>This is to guarantee convergence. Depending on ''r'', the series may also converge sometimes when <span class="texhtml">|''x''|&nbsp;=&nbsp;|''y''|</span>.</ref> and ''r'' is any complex number, one has

:<math>
\begin{align}
(x+y)^r & =\sum_{k=0}^\infty {r \choose k} x^{r-k} y^k \\
& = x^r + r x^{r-1} y + \frac{r(r-1)}{2!} x^{r-2} y^2 + \frac{r(r-1)(r-2)}{3!} x^{r-3} y^3 + \cdots.
\end{align}
</math>
When ''r'' is a nonnegative integer, the binomial coefficients for {{math|1=''k''&nbsp;>&nbsp;''r''}} are zero, so this equation reduces to the usual binomial theorem, and there are at most {{math|1=''r''&nbsp;+&nbsp;1}} nonzero terms. For other values of ''r'', the series typically has infinitely many nonzero terms.

For example, {{math|1=''r''&nbsp;=&nbsp;1/2}} gives the following series for the square root:
:<math>\sqrt{1+x} = 1 + \frac{1}{2}x - \frac{1}{8}x^2 + \frac{1}{16}x^3 - \frac{5}{128}x^4 + \frac{7}{256}x^5 - \cdots</math>

Taking <math>r=-1</math>, the generalized binomial series gives the [[Geometric series#Formula|geometric series formula]], valid for <math>|x| < 1</math>:

:<math>(1+x)^{-1} = \frac{1}{1+x} = 1 - x + x^2 - x^3 + x^4 - x^5 + \cdots</math>

More generally, with {{math|1=''r''&nbsp;=&nbsp;−''s''}}:

:<math>\frac{1}{(1-x)^s} = \sum_{k=0}^\infty {s+k-1 \choose k} x^k.</math>

So, for instance, when <math>s=1/2</math>,
:<math>\frac{1}{\sqrt{1+x}} = 1 -\frac{1}{2}x + \frac{3}{8}x^2 - \frac{5}{16}x^3 + \frac{35}{128}x^4 - \frac{63}{256}x^5 + \cdots</math>

=== Further generalizations ===
The generalized binomial theorem can be extended to the case where ''x'' and ''y'' are complex numbers. For this version, one should again assume <span class="texhtml">|''x''|&nbsp;>&nbsp;|''y''|</span><ref name=convergence group=Note /> and define the powers of {{math|1=''x''&nbsp;+&nbsp;''y''}} and ''x'' using a [[Holomorphic function|holomorphic]] [[complex logarithm|branch of log]] defined on an open disk of radius |''x''| centered at ''x''.
The generalized binomial theorem is valid also for elements ''x'' and ''y'' of a [[Banach algebra]] as long as {{math|1=''xy''&nbsp;=&nbsp;''yx''}}, ''x''&nbsp;is invertible, and&nbsp;<span class="texhtml">||''y/x''||&nbsp;<&nbsp;1</span>.

A version of the binomial theorem is valid for the following [[Pochhammer symbol]]-like family of polynomials: for a given real constant ''c'', define <math> x^{(0)} = 1 </math> and <math> x^{(n)} = \prod_{k=1}^{n}[x+(k-1)c]</math> for <math> n > 0</math>. Then<ref name="Sokolowsky">{{cite journal|url=https://cms.math.ca/crux/backfile/Crux_v5n02_Feb.pdf#page=26|title=Problem 352|first1=Dan|last1=Sokolowsky|first2=Basil C.|last2=Rennie|journal=Crux Mathematicorum|volume=5|issue=2|date=February 1979|pp=55-56}}</ref>
<math display = "block"> (a + b)^{(n)} = \sum_{k=0}^{n}\binom{n}{k}a^{(n-k)}b^{(k)}.</math>
The case {{math|''c'' {{=}} 0}} recovers the usual binomial theorem.

More generally, a sequence <math>\{p_n\}_{n=0}^\infty</math> of polynomials is said to be '''binomial''' if
* <math> \deg p_n = n </math> for all <math>n</math>,
* <math> p_0(0) = 1 </math>, and
* <math> p_n(x+y) = \sum_{k=0}^n \binom{n}{k} p_k(x) p_{n-k}(y) </math> for all <math>x</math>, <math>y</math>, and <math>n</math>.
An operator <math>Q</math> on the space of polynomials is said to be the ''basis operator'' of the sequence <math>\{p_n\}_{n=0}^\infty</math> if <math>Qp_0 = 0</math> and <math> Q p_n = n p_{n-1} </math> for all <math> n \geqslant 1 </math>. A sequence <math>\{p_n\}_{n=0}^\infty</math> is binomial if and only if its basis operator is a [[Delta operator]].<ref>{{cite book |last1=Aigner |first1=Martin |title=Combinatorial Theory |orig-year=Reprint of the 1979 Edition |date=1997 |publisher=Springer |isbn=3-540-61787-6 |page=105}}</ref> Writing <math> E^a </math> for the shift by <math> a </math> operator, the Delta operators corresponding to the above "Pochhammer" families of polynomials are the backward difference <math> I - E^{-c} </math> for <math> c>0 </math>, the ordinary derivative for <math> c=0 </math>, and the forward difference <math> E^{-c} - I </math> for <math> c<0 </math>.

=== Multinomial theorem ===
{{Main|Multinomial theorem}}
The binomial theorem can be generalized to include powers of sums with more than two terms. The general version is

:<math>(x_1 + x_2 + \cdots + x_m)^n
= \sum_{k_1+k_2+\cdots +k_m = n} {n \choose k_1, k_2, \ldots, k_m}
x_1^{k_1} x_2^{k_2} \cdots x_m^{k_m}. </math>

where the summation is taken over all sequences of nonnegative integer indices ''k''<sub>1</sub> through ''k''<sub>''m''</sub> such that the sum of all ''k''<sub>''i''</sub> is&nbsp;''n''. (For each term in the expansion, the exponents must add up to&nbsp;''n''). The coefficients <math> \tbinom n{k_1,\cdots,k_m} </math> are known as multinomial coefficients, and can be computed by the formula

:<math> {n \choose k_1, k_2, \ldots, k_m} = \frac{n!}{k_1! \cdot k_2! \cdots k_m!}.</math>

Combinatorially, the multinomial coefficient <math>\tbinom n{k_1,\cdots,k_m}</math> counts the number of different ways to [[Partition of a set|partition]] an ''n''-element set into [[Disjoint sets|disjoint]] [[subset]]s of sizes {{math|1=''k''<sub>1</sub>,&nbsp;...,&nbsp;''k''<sub>''m''</sub>}}.

=== {{anchor|multi-binomial}} Multi-binomial theorem ===
It is often useful when working in more dimensions, to deal with products of binomial expressions. By the binomial theorem this is equal to

:<math> (x_{1}+y_{1})^{n_{1}}\dotsm(x_{d}+y_{d})^{n_{d}} = \sum_{k_{1}=0}^{n_{1}}\dotsm\sum_{k_{d}=0}^{n_{d}} \binom{n_{1}}{k_{1}}\, x_{1}^{k_{1}}y_{1}^{n_{1}-k_{1}}\;\dotsc\;\binom{n_{d}}{k_{d}}\, x_{d}^{k_{d}}y_{d}^{n_{d}-k_{d}}. </math>

This may be written more concisely, by [[multi-index notation]], as

:<math> (x+y)^\alpha = \sum_{\nu \le \alpha} \binom{\alpha}{\nu} x^\nu y^{\alpha - \nu}.</math>

=== General Leibniz rule ===
{{Main|General Leibniz rule}}

The general Leibniz rule gives the {{mvar|n}}th derivative of a product of two functions in a form similar to that of the binomial theorem:<ref>{{cite book |last=Seely |first=Robert T. |title=Calculus of One and Several Variables |location=Glenview |publisher=Scott, Foresman |year=1973 |isbn=978-0-673-07779-0 }}</ref>

:<math>(fg)^{(n)}(x) = \sum_{k=0}^n \binom{n}{k} f^{(n-k)}(x) g^{(k)}(x).</math>

Here, the superscript {{math|(''n'')}} indicates the {{mvar|n}}th derivative of a function. If one sets {{math|1=''f''(''x'') = ''e''{{sup|''ax''}}}} and {{math|1=''g''(''x'') = ''e''{{sup|''bx''}}}}, and then cancels the common factor of {{math|''e''{{sup|(''a'' + ''b'')''x''}}}} from both sides of the result, the ordinary binomial theorem is recovered.

== Applications ==

=== Multiple-angle identities ===
For the [[complex numbers]] the binomial theorem can be combined with [[de Moivre's formula]] to yield [[List of trigonometric identities#Multiple-angle formulae|multiple-angle formulas]] for the [[sine]] and [[cosine]]. According to De Moivre's formula,
:<math>\cos\left(nx\right)+i\sin\left(nx\right) = \left(\cos x+i\sin x\right)^n.</math>

Using the binomial theorem, the expression on the right can be expanded, and then the real and imaginary parts can be taken to yield formulas for cos(''nx'') and sin(''nx''). For example, since
:<math>\left(\cos x+i\sin x\right)^2 = \cos^2 x + 2i \cos x \sin x - \sin^2 x,</math>
De Moivre's formula tells us that
:<math>\cos(2x) = \cos^2 x - \sin^2 x \quad\text{and}\quad\sin(2x) = 2 \cos x \sin x,</math>
which are the usual double-angle identities. Similarly, since
:<math>\left(\cos x+i\sin x\right)^3 = \cos^3 x + 3i \cos^2 x \sin x - 3 \cos x \sin^2 x - i \sin^3 x,</math>
De Moivre's formula yields
:<math>\cos(3x) = \cos^3 x - 3 \cos x \sin^2 x \quad\text{and}\quad \sin(3x) = 3\cos^2 x \sin x - \sin^3 x.</math>
In general,
:<math>\cos(nx) = \sum_{k\text{ even}} (-1)^{k/2} {n \choose k}\cos^{n-k} x \sin^k x</math>
and
:<math>\sin(nx) = \sum_{k\text{ odd}} (-1)^{(k-1)/2} {n \choose k}\cos^{n-k} x \sin^k x.</math>

=== Series for ''e'' ===
The [[e (mathematical constant)|number ''e'']] is often defined by the formula

:<math>e = \lim_{n\to\infty} \left(1 + \frac{1}{n}\right)^n.</math>

Applying the binomial theorem to this expression yields the usual [[infinite series]] for ''e''. In particular:

:<math>\left(1 + \frac{1}{n}\right)^n = 1 + {n \choose 1}\frac{1}{n} + {n \choose 2}\frac{1}{n^2} + {n \choose 3}\frac{1}{n^3} + \cdots + {n \choose n}\frac{1}{n^n}.</math>

The ''k''th term of this sum is

:<math>{n \choose k}\frac{1}{n^k} = \frac{1}{k!}\cdot\frac{n(n-1)(n-2)\cdots (n-k+1)}{n^k}</math>

As ''n''&nbsp;→&nbsp;∞, the rational expression on the right approaches one, and therefore

:<math>\lim_{n\to\infty} {n \choose k}\frac{1}{n^k} = \frac{1}{k!}.</math>

This indicates that ''e'' can be written as a series:

:<math>e=\sum_{k=0}^\infty\frac{1}{k!}=\frac{1}{0!} + \frac{1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \cdots.</math>

Indeed, since each term of the binomial expansion is an [[Monotonic function|increasing function]] of ''n'', it follows from the [[monotone convergence theorem]] for series that the sum of this infinite series is equal to&nbsp;''e''.

=== Probability ===
The binomial theorem is closely related to the probability mass function of the [[negative binomial distribution]]. The probability of a (countable) collection of independent Bernoulli trials <math>\{X_t\}_{t\in S}</math> with probability of success <math>p\in [0,1]</math> all not happening is <math display="block"> P\left(\bigcap_{t\in S} X_t^C\right) = (1-p)^{|S|} = \sum_{n=0}^{|S|} {|S| \choose n} (-p)^n</math>A useful upper bound for this quantity is <math> e^{-pn}</math>. <ref>{{Cite book|title=Data Compression|last=Cover|first=Thomas M.|last2=Thomas|first2=Joy A.|date=2001-01-01|publisher=John Wiley & Sons, Inc.|isbn=9780471200611|pages=320|language=en|doi=10.1002/0471200611.ch5}}</ref>

== The binomial theorem in abstract algebra ==

Formula (1) is valid more generally for any elements ''x'' and ''y'' of a [[semiring]] satisfying {{math|1=''xy''&nbsp;=&nbsp;''yx''}}. The [[theorem]] is true even more generally: [[alternativity]] suffices in place of [[associativity]].

The binomial theorem can be stated by saying that the [[polynomial sequence]] {{math|1={{mset|&nbsp;1,&nbsp;''x'',&nbsp;''x''<sup>2</sup>,&nbsp;''x''<sup>3</sup>,&nbsp;...&nbsp;}}}} is of [[binomial type]].

== In popular culture ==
* The binomial theorem is mentioned in the [[Major-General's Song]] in the comic opera [[The Pirates of Penzance]].
* [[Professor Moriarty]] is described by Sherlock Holmes as having written [[A Treatise on the Binomial Theorem|a treatise on the binomial theorem]].
* The Portuguese poet [[Fernando Pessoa]], using the heteronym [[Álvaro de Campos]], wrote that "Newton's Binomial is as beautiful as the [[Venus de Milo]]. The truth is that few people notice it."<ref>{{cite web|url=http://arquivopessoa.net/textos/224|title=Arquivo Pessoa: Obra Édita - O binómio de Newton é tão belo como a Vénus de Milo.|publisher=arquivopessoa.net}}</ref>
* In the 2014 film [[The Imitation Game]], Alan Turing makes reference to Isaac Newton's work on the Binomial Theorem during his first meeting with Commander Denniston at Bletchley Park.

== See also ==
{{portal|Mathematics}}
* [[Binomial approximation]]
* [[Binomial distribution]]
* [[Binomial inverse theorem]]
* [[Stirling's approximation]]

== Notes ==
{{reflist|group=Note}}

== References ==
{{reflist|30em}}

== Further reading ==
* {{cite journal|last=Bag|first=Amulya Kumar|year=1966|title=Binomial theorem in ancient India|journal=Indian J. History Sci|volume=1|issue=1|pages=68–74}}
* {{cite book|last1=Graham|first1=Ronald|first2=Donald |last2=Knuth|first3= Oren|last3= Patashnik|title=Concrete Mathematics|publisher=Addison Wesley|year=1994|edition=2nd|pages=153–256|chapter=(5) Binomial Coefficients|isbn=978-0-201-55802-9|oclc=17649857}}

== External links ==
{{Wikibooks|Combinatorics|Binomial Theorem|The Binomial Theorem}}
* {{SpringerEOM|id=Newton_binomial|first=E.D.|last= Solomentsev|title=Newton binomial}}
* [http://demonstrations.wolfram.com/BinomialTheorem/ Binomial Theorem] by [[Stephen Wolfram]], and [http://demonstrations.wolfram.com/BinomialTheoremStepByStep/ "Binomial Theorem (Step-by-Step)"] by Bruce Colletti and Jeff Bryant, [[Wolfram Demonstrations Project]], 2007.

{{PlanetMath attribution|id=338|title=inductive proof of binomial theorem}}

{{Authority control}}

[[Category:Factorial and binomial topics]]
[[Category:Theorems in algebra]]
[[Category:Articles containing proofs]]

১৬:০৪, ১৩ জুলাই ২০১৯ তারিখে সংশোধিত সংস্করণ

The binomial coefficient appears as the bth entry in the nth row of Pascal's triangle (counting starts at ). Each entry is the sum of the two above it.

প্রাথমিক বীজগণিতে, দ্বিপদী উপপাদ্য (বা দ্বিপদী বিস্তার ) একটি দ্বিপদী রাশির সূচকের বীজগাণিতিক সম্প্রসারণ বর্ণনা করে। এই উপপাদ্য অনুযায়ী, একটি (x + y)n আকারের বহুপদীকে কয়েকটি a xbyc আকারের রাশির সমষ্টি রূপে প্রকাশ করা সম্ভব, যেখানে b এবং c সূচকদ্বয় প্রত্যেকে অঋণাত্মক পূর্ণসংখ্যাb + c = n, এবং প্রতিটি রাশির সহগ a একটি নির্দিষ্ট ধনাত্মক পূর্ণসংখ্যা যার মান nb এর উপর নির্ভর করে। উদাহরণস্বরূপ, n = 4 এর জন্য-

The coefficient a in the term of a xbyc is known as the binomial coefficient or (the two have the same value). These coefficients for varying n and b can be arranged to form Pascal's triangle. These numbers also arise in combinatorics, where gives the number of different combinations of b elements that can be chosen from an n-element set. Therefore is often pronounced as "n choose b".

History

Special cases of the binomial theorem were known since at least the 4th century BC when Greek mathematician Euclid mentioned the special case of the binomial theorem for exponent 2.[১][২] There is evidence that the binomial theorem for cubes was known by the 6th century AD in India.[১][২]

Binomial coefficients, as combinatorial quantities expressing the number of ways of selecting k objects out of n without replacement, were of interest to ancient Indian mathematicians. The earliest known reference to this combinatorial problem is the Chandaḥśāstra by the Indian lyricist Pingala (c. 200 BC), which contains a method for its solution.[৩]:২৩০ The commentator Halayudha from the 10th century AD explains this method using what is now known as Pascal's triangle.[৩] By the 6th century AD, the Indian mathematicians probably knew how to express this as a quotient ,[৪] and a clear statement of this rule can be found in the 12th century text Lilavati by Bhaskara.[৪]

The first formulation of the binomial theorem and the table of binomial coefficients, to our knowledge, can be found in a work by Al-Karaji, quoted by Al-Samaw'al in his "al-Bahir".[৫][৬][৭] Al-Karaji described the triangular pattern of the binomial coefficients[৮] and also provided a mathematical proof of both the binomial theorem and Pascal's triangle, using an early form of mathematical induction.[৮] The Persian poet and mathematician Omar Khayyam was probably familiar with the formula to higher orders, although many of his mathematical works are lost.[২] The binomial expansions of small degrees were known in the 13th century mathematical works of Yang Hui[৯] and also Chu Shih-Chieh.[২] Yang Hui attributes the method to a much earlier 11th century text of Jia Xian, although those writings are now also lost.[৩]:১৪২

In 1544, Michael Stifel introduced the term "binomial coefficient" and showed how to use them to express in terms of , via "Pascal's triangle".[১০] Blaise Pascal studied the eponymous triangle comprehensively in the treatise Traité du triangle arithmétique (1653). However, the pattern of numbers was already known to the European mathematicians of the late Renaissance, including Stifel, Niccolò Fontana Tartaglia, and Simon Stevin.[১০]

Isaac Newton is generally credited with the generalized binomial theorem, valid for any rational exponent.[১০][১১]

Theorem statement

According to the theorem, it is possible to expand any power of x + y into a sum of the form

where each is a specific positive integer known as a binomial coefficient. (When an exponent is zero, the corresponding power expression is taken to be 1 and this multiplicative factor is often omitted from the term. Hence one often sees the right side written as .) This formula is also referred to as the binomial formula or the binomial identity. Using summation notation, it can be written as

The final expression follows from the previous one by the symmetry of x and y in the first expression, and by comparison it follows that the sequence of binomial coefficients in the formula is symmetrical. A simple variant of the binomial formula is obtained by substituting 1 for y, so that it involves only a single variable. In this form, the formula reads

or equivalently

Examples

The most basic example of the binomial theorem is the formula for the square of x + y:

The binomial coefficients 1, 2, 1 appearing in this expansion correspond to the second row of Pascal's triangle. (Note that the top "1" of the triangle is considered to be row 0, by convention.) The coefficients of higher powers of x + y correspond to lower rows of the triangle:

Several patterns can be observed from these examples. In general, for the expansion (x + y)n:

  1. the powers of x start at n and decrease by 1 in each term until they reach 0 (with x0 = 1, often unwritten);
  2. the powers of y start at 0 and increase by 1 until they reach n;
  3. the nth row of Pascal's Triangle will be the coefficients of the expanded binomial when the terms are arranged in this way;
  4. the number of terms in the expansion before like terms are combined is the sum of the coefficients and is equal to 2n; and
  5. there will be n + 1 terms in the expression after combining like terms in the expansion.

The binomial theorem can be applied to the powers of any binomial. For example,

For a binomial involving subtraction, the theorem can be applied by using the form (xy)n = (x + (−y))n. This has the effect of changing the sign of every other term in the expansion:

Geometric explanation

Visualisation of binomial expansion up to the 4th power

For positive values of a and b, the binomial theorem with n = 2 is the geometrically evident fact that a square of side a + b can be cut into a square of side a, a square of side b, and two rectangles with sides a and b. With n = 3, the theorem states that a cube of side a + b can be cut into a cube of side a, a cube of side b, three a×a×b rectangular boxes, and three a×b×b rectangular boxes.

In calculus, this picture also gives a geometric proof of the derivative [১২] if one sets and interpreting b as an infinitesimal change in a, then this picture shows the infinitesimal change in the volume of an n-dimensional hypercube, where the coefficient of the linear term (in ) is the area of the n faces, each of dimension

Substituting this into the definition of the derivative via a difference quotient and taking limits means that the higher order terms, and higher, become negligible, and yields the formula interpreted as

"the infinitesimal rate of change in volume of an n-cube as side length varies is the area of n of its -dimensional faces".

If one integrates this picture, which corresponds to applying the fundamental theorem of calculus, one obtains Cavalieri's quadrature formula, the integral – see proof of Cavalieri's quadrature formula for details.[১২]

Binomial coefficients

The coefficients that appear in the binomial expansion are called binomial coefficients. These are usually written , and pronounced “n choose k”.

Formulae

The coefficient of xnkyk is given by the formula

which is defined in terms of the factorial function n!. Equivalently, this formula can be written

with k factors in both the numerator and denominator of the fraction. Note that, although this formula involves a fraction, the binomial coefficient is actually an integer.

Combinatorial interpretation

The binomial coefficient can be interpreted as the number of ways to choose k elements from an n-element set. This is related to binomials for the following reason: if we write (x + y)n as a product

then, according to the distributive law, there will be one term in the expansion for each choice of either x or y from each of the binomials of the product. For example, there will only be one term xn, corresponding to choosing x from each binomial. However, there will be several terms of the form xn−2y2, one for each way of choosing exactly two binomials to contribute a y. Therefore, after combining like terms, the coefficient of xn−2y2 will be equal to the number of ways to choose exactly 2 elements from an n-element set.

Proofs

Combinatorial proof

Example

The coefficient of xy2 in

equals because there are three x,y strings of length 3 with exactly two y's, namely,

corresponding to the three 2-element subsets of { 1, 2, 3 }, namely,

where each subset specifies the positions of the y in a corresponding string.

General case

Expanding (x + y)n yields the sum of the 2 n products of the form e1e2 ... e n where each e i is x or y. Rearranging factors shows that each product equals xnkyk for some k between 0 and n. For a given k, the following are proved equal in succession:

  • the number of copies of xn − kyk in the expansion
  • the number of n-character x,y strings having y in exactly k positions
  • the number of k-element subsets of { 1, 2, ..., n}
  • (this is either by definition, or by a short combinatorial argument if one is defining as ).

This proves the binomial theorem.

Inductive proof

Induction yields another proof of the binomial theorem. When n = 0, both sides equal 1, since x0 = 1 and . Now suppose that the equality holds for a given n; we will prove it for n + 1. For jk ≥ 0, let [ƒ(xy)] j,k denote the coefficient of xjyk in the polynomial ƒ(xy). By the inductive hypothesis, (x + y)n is a polynomial in x and y such that [(x + y)n] j,k is if j + k = n, and 0 otherwise. The identity

shows that (x + y)n + 1 also is a polynomial in x and y, and

since if j + k = n + 1, then (j − 1) + k = n and j + (k − 1) = n. Now, the right hand side is

by Pascal's identity.[১৩] On the other hand, if j +k ≠ n + 1, then (j – 1) + k ≠ n and j +(k – 1) ≠ n, so we get 0 + 0 = 0. Thus

which is the inductive hypothesis with n + 1 substituted for n and so completes the inductive step.

Generalizations

Newton's generalized binomial theorem

Around 1665, Isaac Newton generalized the binomial theorem to allow real exponents other than nonnegative integers. (The same generalization also applies to complex exponents.) In this generalization, the finite sum is replaced by an infinite series. In order to do this, one needs to give meaning to binomial coefficients with an arbitrary upper index, which cannot be done using the usual formula with factorials. However, for an arbitrary number r, one can define

where is the Pochhammer symbol, here standing for a falling factorial. This agrees with the usual definitions when r is a nonnegative integer. Then, if x and y are real numbers with |x| > |y|,[Note ১] and r is any complex number, one has

When r is a nonnegative integer, the binomial coefficients for k > r are zero, so this equation reduces to the usual binomial theorem, and there are at most r + 1 nonzero terms. For other values of r, the series typically has infinitely many nonzero terms.

For example, r = 1/2 gives the following series for the square root:

Taking , the generalized binomial series gives the geometric series formula, valid for :

More generally, with r = −s:

So, for instance, when ,

Further generalizations

The generalized binomial theorem can be extended to the case where x and y are complex numbers. For this version, one should again assume |x| > |y|[Note ১] and define the powers of x + y and x using a holomorphic branch of log defined on an open disk of radius |x| centered at x. The generalized binomial theorem is valid also for elements x and y of a Banach algebra as long as xy = yx, x is invertible, and ||y/x|| < 1.

A version of the binomial theorem is valid for the following Pochhammer symbol-like family of polynomials: for a given real constant c, define and for . Then[১৪]

The case c = 0 recovers the usual binomial theorem.

More generally, a sequence of polynomials is said to be binomial if

  • for all ,
  • , and
  • for all , , and .

An operator on the space of polynomials is said to be the basis operator of the sequence if and for all . A sequence is binomial if and only if its basis operator is a Delta operator.[১৫] Writing for the shift by operator, the Delta operators corresponding to the above "Pochhammer" families of polynomials are the backward difference for , the ordinary derivative for , and the forward difference for .

Multinomial theorem

The binomial theorem can be generalized to include powers of sums with more than two terms. The general version is

where the summation is taken over all sequences of nonnegative integer indices k1 through km such that the sum of all ki is n. (For each term in the expansion, the exponents must add up to n). The coefficients are known as multinomial coefficients, and can be computed by the formula

Combinatorially, the multinomial coefficient counts the number of different ways to partition an n-element set into disjoint subsets of sizes k1, ..., km.

Multi-binomial theorem

It is often useful when working in more dimensions, to deal with products of binomial expressions. By the binomial theorem this is equal to

This may be written more concisely, by multi-index notation, as

General Leibniz rule

The general Leibniz rule gives the nth derivative of a product of two functions in a form similar to that of the binomial theorem:[১৬]

Here, the superscript (n) indicates the nth derivative of a function. If one sets f(x) = eax and g(x) = ebx, and then cancels the common factor of e(a + b)x from both sides of the result, the ordinary binomial theorem is recovered.

Applications

Multiple-angle identities

For the complex numbers the binomial theorem can be combined with de Moivre's formula to yield multiple-angle formulas for the sine and cosine. According to De Moivre's formula,

Using the binomial theorem, the expression on the right can be expanded, and then the real and imaginary parts can be taken to yield formulas for cos(nx) and sin(nx). For example, since

De Moivre's formula tells us that

which are the usual double-angle identities. Similarly, since

De Moivre's formula yields

In general,

and

Series for e

The number e is often defined by the formula

Applying the binomial theorem to this expression yields the usual infinite series for e. In particular:

The kth term of this sum is

As n → ∞, the rational expression on the right approaches one, and therefore

This indicates that e can be written as a series:

Indeed, since each term of the binomial expansion is an increasing function of n, it follows from the monotone convergence theorem for series that the sum of this infinite series is equal to e.

Probability

The binomial theorem is closely related to the probability mass function of the negative binomial distribution. The probability of a (countable) collection of independent Bernoulli trials with probability of success all not happening is

A useful upper bound for this quantity is . [১৭]

The binomial theorem in abstract algebra

Formula (1) is valid more generally for any elements x and y of a semiring satisfying xy = yx. The theorem is true even more generally: alternativity suffices in place of associativity.

The binomial theorem can be stated by saying that the polynomial sequence { 1, xx2x3, ... } is of binomial type.

In popular culture

See also

Notes

  1. This is to guarantee convergence. Depending on r, the series may also converge sometimes when |x| = |y|.

References

  1. Weisstein, Eric W.। "Binomial Theorem"Wolfram MathWorld 
  2. Coolidge, J. L. (১৯৪৯)। "The Story of the Binomial Theorem"। The American Mathematical Monthly56 (3): 147–157। জেস্টোর 2305028ডিওআই:10.2307/2305028 
  3. Jean-Claude Martzloff; S.S. Wilson; J. Gernet; J. Dhombres (১৯৮৭)। A history of Chinese mathematics। Springer। 
  4. Biggs, N. L. (১৯৭৯)। "The roots of combinatorics"। Historia Math.6 (2): 109–136। ডিওআই:10.1016/0315-0860(79)90074-0 
  5. "THE BINOMIAL THEOREM : A WIDESPREAD CONCEPT IN MEDIEVAL ISLAMIC MATHEMATICS" (পিডিএফ)core.ac.uk। পৃষ্ঠা 401। সংগ্রহের তারিখ ২০১৯-০১-০৮ 
  6. "Taming the unknown. A history of algebra from antiquity to the early twentieth century" (পিডিএফ)Bulletin of the American Mathematical Society: 727। However, algebra advanced in other respects. Around 1000, al-Karaji stated the binomial theorem 
  7. Rashed, R. (১৯৯৪-০৬-৩০)। The Development of Arabic Mathematics: Between Arithmetic and Algebra (ইংরেজি ভাষায়)। Springer Science & Business Media। পৃষ্ঠা 63। আইএসবিএন 9780792325659 
  8. ও'কনর, জন জে.; রবার্টসন, এডমুন্ড এফ., "Abu Bekr ibn Muhammad ibn al-Husayn Al-Karaji", ম্যাকটিউটর গণিতের ইতিহাস আর্কাইভ, সেন্ট অ্যান্ড্রুজ বিশ্ববিদ্যালয় 
  9. Landau, James A. (১৯৯৯-০৫-০৮)। "Historia Matematica Mailing List Archive: Re: [HM] Pascal's Triangle" (mailing list email)Archives of Historia Matematica। সংগ্রহের তারিখ ২০০৭-০৪-১৩ 
  10. Kline, Morris (১৯৭২)। History of mathematical thought। Oxford University Press। পৃষ্ঠা 273। 
  11. Bourbaki, N. (১৮ নভেম্বর ১৯৯৮)। Elements of the History of Mathematics Paperback। J. Meldrum (Translator)। আইএসবিএন 978-3-540-64767-6 
  12. Barth, Nils R. (২০০৪)। "Computing Cavalieri's Quadrature Formula by a Symmetry of the n-Cube"। The American Mathematical Monthly111 (9): 811–813। আইএসএসএন 0002-9890জেস্টোর 4145193ডিওআই:10.2307/4145193, author's copy, further remarks and resources 
  13. Binomial theorem – inductive proofs ওয়েব্যাক মেশিনে আর্কাইভকৃত ফেব্রুয়ারি ২৪, ২০১৫ তারিখে
  14. Sokolowsky, Dan; Rennie, Basil C. (ফেব্রুয়ারি ১৯৭৯)। "Problem 352" (পিডিএফ)Crux Mathematicorum5 (2): 55–56। 
  15. Aigner, Martin (১৯৯৭) [Reprint of the 1979 Edition]। Combinatorial Theory। Springer। পৃষ্ঠা 105। আইএসবিএন 3-540-61787-6 
  16. Seely, Robert T. (১৯৭৩)। Calculus of One and Several Variables। Glenview: Scott, Foresman। আইএসবিএন 978-0-673-07779-0 
  17. Cover, Thomas M.; Thomas, Joy A. (২০০১-০১-০১)। Data Compression (ইংরেজি ভাষায়)। John Wiley & Sons, Inc.। পৃষ্ঠা 320। আইএসবিএন 9780471200611ডিওআই:10.1002/0471200611.ch5 
  18. "Arquivo Pessoa: Obra Édita - O binómio de Newton é tão belo como a Vénus de Milo."। arquivopessoa.net। 

Further reading

  • Bag, Amulya Kumar (১৯৬৬)। "Binomial theorem in ancient India"। Indian J. History Sci1 (1): 68–74। 
  • Graham, Ronald; Knuth, Donald; Patashnik, Oren (১৯৯৪)। "(5) Binomial Coefficients"। Concrete Mathematics (2nd সংস্করণ)। Addison Wesley। পৃষ্ঠা 153–256। আইএসবিএন 978-0-201-55802-9ওসিএলসি 17649857 

External links

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