I have some work and im confused on these problems... I did all i can (looked in book and such) and need help.

Simplify the expression cos (X-5pie /2)

For this one i know that you can go to (cosx)(cos-5pie/2) + (sinx)(sin-5pie/2) But i dont know what to do from there

second isIf cotx= 4/3 and 180degrees< x < 270degrees, find the exact values of sin2x and cos2x

Here, i know if there was a triangle, there would be the 5,4,and3 to it, but i dont know how you go to the sin2x and cos2x

third isFind the solutions of sin2x = cosx that are in interval [0, 2pie)

If anyone could help, youd be my hero. I have 8 tests this week and im stressing out on these problems....