Differentiating under the integral

ddcamp
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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?
 
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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.
 
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