Primitive of Power of x by Exponential of a x/Examples/x cubed by e^-x

Example of Use of Primitive of $x^n e^{a x}$

 * $\ds \int x^3 e^{-x} \rd x = -e^{-x} \paren {x^3 + 3 x^2 + 6 x + 6} + C$

Proof
From Primitive of $x^n e^{a x}$:

Hence: