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Use integration by parts twice on $ \int e^{xy} \cos(y) dy$

i wanna integrate $\cos(y)$ first and derivate $e^{xy}$ since x is a constant of the integration.

we have $e^{xy} \sin(y)$ + $ \int xe^{xy} \sin(y) dy$

followed by

$e^{xy} \sin(y)$ + $ xe^{xy} \cos(y) dy$ + $ \int -x^{2}e^{xy} \cos(y) dy$

This doesnt look right....

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1 Answer 1

up vote 2 down vote accepted

It looks right.

Let $$I=\int e^{xy}\cos ydy$$ Then, as you have calculated, $$I=e^{xy}\sin y+xe^{xy}\cos y-x^2I$$ Therefore, $$I=\frac{e^{xy}\sin y+xe^{xy}\cos y}{1+x^2}$$

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wow really? i haven't done an integration by parts in years and i couldn't even find my textbook with the formula! thx i couldnt remember the trick from there tyvm –  Faust7 Mar 22 '13 at 2:20

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