# How to show that it's equal (Intergral)

• Aug 16th 2008, 05:59 PM
noppawit
How to show that it's equal (Intergral)
Given that m and n are positive integers, show that

int [from 0 to 1] (x^m)(1-x)^n dx = int [from 0 to 1] (x^n)(1-x)^m dx
• Aug 16th 2008, 07:54 PM
mr fantastic
Quote:

Originally Posted by noppawit
Given that m and n are positive integers, show that

int [from 0 to 1] (x^m)(1-x)^n dx = int [from 0 to 1] (x^n)(1-x)^m dx

There are numerous approaches to show that each integral is equal to $\frac{n! m!}{(n + m + 1)!}$. Here is one approach:

Using integration by parts:

$\beta (m, n) = \int_{0}^{1} x^m (1 - x)^n \, dx = \frac{n}{m+1} \int_{0}^{1} x^{m+1} (1 - x)^{n-1} \, dx$.

In fact, using integration by parts t times you get:

$\beta (m, n) = \frac{n (n-1) (n-2) \, .... \, (n - [t-1])}{(m+1) (m+2) (m+3) \, .... \, (m+t)} \int_{0}^{1} x^{m+t} (1 - x)^{n-t} \, dx$.

Let t = n:

$\beta (m, n) = \frac{n (n-1) (n-2) \, .... \, (1)}{(m+1) (m+2) (m+3) \, .... \, (m+t)} \int_{0}^{1} x^{m+n} \, dx$.

The final integration is simple.

From the symmetry of the answer, it's clear that $\int_{0}^{1} x^n (1 - x)^m \, dx$ has the same value.
• Aug 17th 2008, 12:27 AM
Coomast
If you use the substitution:

$1-x=t$

on the left hand side, you will almost immediately find the right hand side of the equality. This might be a bit easier than mr fantastic's (perfectly valid) solution.