I was asked this question can someone please help I do not know where to start

Find the sum to n terms:

a) cosΘ + cos3Θ + cos5Θ + ...

the answer should be (-cos((2n+1)Θ)+cos((2n-1)Θ))/(2(1 - cos2Θ))

I don't know hoe to go about answering this question.

Please Help