Thank you so much for your quick answer for a). I had forgot you could make 1 = sin t + cos t
Anyways can you check to see if I did b) correctly?
b) cos 2t = (cos t)^2 - (sin t)^2
cos 2t = -sin^2t + cos^2t
cos^2t -sin^2t
(1-sin^2t) + (cos^2t -1)
cos 2t = -sin^2t + cos^2t