Solve that equation : `costheta+cos3theta-2cos2theta=0`

Explanation

`costheta+cos3theta -2cos2theta = 0`

`=>2cos((theta+3theta)/2)cos((theta-3theta)/2)-2cos2theta = 0`

`=>2cos 2thetacostheta-2cos 2theta = 0`

`=>2cos2theta(costheta -1) = 0`

`=>cos2theta = 0 and costheta = 1`

`=>2theta = 2npi+-pi/2 and theta = 2mpi+-0`

`=>theta = npi+-pi/4 and theta = 2mpi`, where `m` and `n` are integers.