We have to find the integral `\int e^{-\theta}cos(2\theta) d\theta`

We can do this by integration by parts i.e.

`\int e^{-\theta}cos(2\theta) d\theta=e^{-\theta}\int cos(2\theta) d\theta -\int( \frac{d}{d\theta}(e^{-\theta})\int cos(2\theta) d\theta )d\theta`

`=e^{-\theta}.\frac{sin(2\theta)}{2}-\int-e^{-\theta}.\frac{sin(2\theta)}{2} d\theta`

(The entire section contains 194 words.)

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