212) solutions are 60,120,22.5,157.5,67.5,112.5
1) add them to get sum of solutions....
the given expression equals 4a^3 + 2 a^2 - 2 a -1 = 0, where a = cos 2∂ ( just simplify using compound angle)
now u have to find values of a satisfying above equation which u will get (-.5), (1/√2) and - (1/√2)
and as ∂ lies between 0 to 180 , 2∂ lies between 0 to 360
so values of ∂ satisfying cos 2∂ =(-.5), (.5) and (1/√2) are the solutions which are mentioned above//