Question

If `costheta=-15/17` and `pi/2<theta<pi`, how do you find `cos2theta`?

Answer 1

`161/289`

Explanation:

Since `cos2theta=cos^2theta-sin^2theta`

and `sin^2theta=1-cos^2theta`, you will have:

`cos2theta=cos^2theta-(1-cos^2theta)=2cos^2theta-1`
`=2(-15/17)^2-1`
`=2*225/289-1`
`=(450-289)/289=161/289`