# Evaluate Integral

1. Sep 7, 2008

### afcwestwarrior

∫x^4 e^-x dx

ok this equation looks like this integral
∫u^n e^au du= (1/a) u^n e^au - n/a ∫u^n-1 e^au du

i did integration by parts and i ran in circles, and i tried substituting u=-x

but i couldn't do much with it, so what can i do

2. Sep 7, 2008

### Focus

$$\int x^4 e^{-x} dx = -x^4 e^{-x} + \int 4 x^3 e^{-x} dx$$
Itterate this for the integral on the right untill the x term disapears.