(a) Prove that $\tan\left[i \log \left(\frac{a-ib}{a+ib}\right)\right…

Previous year question for Engineering Mathematics-I.

Question

(a) Prove that $\tan\left[i \log \left(\frac{a-ib}{a+ib}\right)\right] = \frac{2ab}{a^2 - b^2}$.

(b) Sum to $n$ terms and to infinity the following series:

$$1 + x \cos \alpha + x^2 \cos 2\alpha + x^3 \cos 3\alpha + \dots$$

where $|x| < 1$.

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