By considering from k=0 to k=n-1, show that
and i also dont know how to put limits in summation :P please tell that too
If you do as suggested the summation can be split into two parts, one real the other imaginary.
The summation itself can be summed easily since it is a GP with common ratio
Do that and then equate reals and imaginaries across the two results.
(Actually, you only need to equate reals.)