Pre Calc Problems

How I can make these pre-calc problems?

Half-angle using appropriate formula to find the value of each expression. 1) cos (pi/12) 2) sin (9pi / 8) Please explain! I'd rather have a detailed explanation as opposed to response. Thanks in advance! However, I'm supposed to find the exact value, how I can do that? I know that pi = 180

Use half angle formulas for sine and cosine. 1) cos (θ / 2) = √ [(1 + cos.theta) / 2] = θ π / 6 sin (θ / 2) = – √ [(1 – cos.theta) / 2] = θ 9π / 4, which is coterminal with π / 4

Pre-Calc problems 3-5 part 2.mp4