yeah i did........ actually we had to it for square and cube, i did for the square but for the cube i can not derive this, i can not prove it using x^3 + y^3 + 3xy^2 + 3yx^2
I'll do no such thing, you'll prove it yourself. Start by substituting "cos(theta)" for x and " i sin(theta) " for y. Simplify and collect your real parts and imaginary parts.