# Math Help - Need help integrating

1. ## Need help integrating

Hi all,

I need help integrating:

intg ((e^t)*cos(n*t))dt

I have tried integrating by parts but it seemed like it just got more complicated. I know there is a theroem but i am not allowed to use it.

ArTiCk

2. You actually integrate by parts twice and end up with a multiple of

intg ((e^t)*cos(n*t))dt on both sides.

Combine these ans solve

In both cases let u = e^t to get started.

Let me know if it works out

3. Originally Posted by ArTiCK
Hi all,

I need help integrating:

intg ((e^t)*cos(n*t))dt

I have tried integrating by parts but it seemed like it just got more complicated. I know there is a theroem but i am not allowed to use it.

$\int e^t \cos(nt)\; dt= \text{Re}\left\{ \int e^t e^{i n t}\; dt\right\}=\text{Re} \left\{\int e^{t(1+in)}\;dt\right\}$