I have absolutely no idea where this question should go, so please feel free to move it to the correct subforum.

a)If , use de Moivre's theorem to show that

b) Express In terms of and

Part a) I have done correctly, but part b) Is confusing me. My working so far:

Then I hit a wall. At first glance, I try to say ' ' but that doesn't fit the question. I see that I have to use the theorem in some way, but I have no idea on the correct approach from here.