Differentiating under the integral

[ddcamp]

Homework Statement

integral 0 to infinite (x^3)(e^-8)(cos(5x)) dx

Homework Equations

integral by part?

The Attempt at a Solution

should I use integral by part?

 

[fzero]

Do you mean

$$\int_0^\infty x^3 e^{-8x} \cos(5x) dx?$$

You might want to use the identity

$$e^{iu} = \cos u + i \sin u$$.

and then integrate by parts.