By considering $\displaystyle \sum(1+\iota \tan \theta)^k$ from k=0 to k=n-1, show that

$\displaystyle \sum \cos k\theta \sec^k \theta = \cot \theta\sin n\theta\sec^n\theta$

stuck here

please help

and i also dont know how to put limits in summation :P please tell that too