# Need help in Integral

• Aug 27th 2012, 04:38 PM
nirmal019
Need help in Integral
∫x^(4)e^(-x^(2)/2)dx in the limit [-∞,∞]

I tried to solve this problem many times but i just couldn't (Headbang). Can someone help me ?
• Aug 27th 2012, 05:43 PM
harish21
Re: Need help in Integral
Quote:

Originally Posted by nirmal019
∫x^(4)e^(-x^(2)/2)dx in the limit [-∞,∞]

I tried to solve this problem many times but i just couldn't (Headbang). Can someone help me ?

write down the even function as

$\displaystyle \int_{-\infty}^{\infty} x^4 e^{-\frac{x^2}{2}} dx$

$\displaystyle =2 \int_0^{\infty} x^4 e^{-\frac{x^2}{2}} dx$

let $\displaystyle u=x^2 \implies du = 2x dx$. also, $\displaystyle x=0 \implies u=0; and x=\infty \implies u=\infty$so you have

$\displaystyle \int_0^{\infty} u^{3/2} e^{-\frac{u}{2}} du$

try solving from here...use gamma function
• Aug 27th 2012, 07:50 PM
nirmal019
Re: Need help in Integral
Thank you so much I have solved the promblem :). Actually I never thought of the Gamma function and trying different methods :P
• Aug 27th 2012, 09:20 PM
MaxJasper
Re: Need help in Integral
Using Laplace transform:

$\displaystyle \mathcal{L}_x\left[\int_{-\infty }^{\infty } x^4 e^{-\frac{x^2}{2}} \, dx\right](s)$ = $\displaystyle \frac{3 \sqrt{2 \pi }}{s}$

Taking inverse Laplace:

$\displaystyle \mathcal{L}_s^{-1}\left[\frac{3 \sqrt{2 \pi }}{s}\right](x)$ = $\displaystyle 3 \sqrt{2 \pi }$