Question

Given `costheta=-15/17` and `180<theta<270`, how do you find `cos2theta`?

Answer 1

`cos 2theta=2cos^2 theta-1=2(-15/17)^2-1=450/289-1=161/289`.

Explanation:

`cos 2theta=2cos^2 theta-1=2(-15/17)^2-1=450/289-1=161/289`.