A choose b

Here is the formula for a choose b: $\binom{a}{b}=\frac{a!}{b!(a-b)!}$ . This is assuming that of course $a \ge b$ .

Uses

a choose b counts the number of ways you can pick b things from a set of a things. For example $\binom{8}{2}=\frac{8!}{2!(8-2)!}=\frac{8(7)}{2}=\frac{42}{2}=21$ . More here.

Choose 2

Here is a list of n choose 2's $\binom{2}{2}=1$ $\binom{3}{2}=3$ $\binom{4}{2}=6$ $\binom{5}{2}=10$ These are triangle numbers! My proof uses induction (assuming something is true unless proved true or false). $\binom{n}{2}=1+2+3...+(n-1)$ Then Simplify: $\frac{n!}{2!(n-2)!}=\frac{n(n+1)}{2}$ Simplify further: $\frac{n(n+1)}{2}=\frac{n(n+1)}{2} \square$ If you don't get what I did on the second step go to Proof Without Words on this wiki.

Pascal's Identity

Main article: Pascal's Identity

Statement

${n \choose k}={n-1\choose k-1}+{n-1\choose k}$

Proof

$\begin{eqnarray*}\binom{n-1}{k-1}+\binom{n-1}{k}&=&\frac{(n-1)!}{(k-1)!(n-k)!}+\frac{(n-1)!}{k!(n-k-1)!}\\ &=&(n-1)!\left(\frac{k}{k!(n-k)!}+\frac{n-k}{k!(n-k)!}\right)\\ &=&(n-1)!\cdot \frac{n}{k!(n-k)!}\\ &=&\frac{n!}{k!(n-k)!}\\ &=&\binom{n}{k}. \qquad\qquad\square\end{eqnarray*}$

Binomial Theorem and Pascal's Triangle

Pascal's triangle is an array of numbers that represent binomial coefficients. It looks like this:

     1
   1  1
  1  2  1
 1  3  3  1
1  4  6  4  1

And so on...

This triangle can be rewritten like this:

$\binom{0}{0}$

$\binom{1}{0} \binom{1}{1}$

$\binom{2}{0} \binom{2}{1} \binom{2}{2}$

$\binom{3}{0} \binom{3}{1} \binom{3}{2} \binom{3}{3}$

I'll encourage you to prove it by yourself

Another way to build it is to start with two diagonal rows of one and then the rest of the numbers are the sum of the two numbers above it.

The zeroth row has a sum of $1=2^0$ . The first row has a sum of $2=2^1$ . The $n^{th}$ row has a sum of $2^n$ . This is because in the second way we build the tr The 1st downward diagonal is a row of 1's, the 2nd downward diagonal on each side consists of the natural numbers, the 3rd diagonal the triangular numbers, and the 4th the pyramidal numbers. There are many interesting applications of pascals triangle, I encourage you to explore the patterns.

This article has been proposed for deletion. The reason given is: unnecessary page, Duplicate of Combination. Sysops: Before deleting this article, please check the article discussion pages and history.

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