# Thread: Prove this by whole cube expansion

1. ## Prove this by whole cube expansion

prove that (cos theta + i sin theta) ^ 3

equals

cos 3theta + i sin 3 theta

by expanding the whole cube

2. ## Re: Prove this by whole cube expansion

Well, have you tried expanding the cube?

3. ## Re: Prove this by whole cube expansion

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

4. ## Re: Prove this by whole cube expansion

so kindly prove it for me

5. ## Re: Prove this by whole cube expansion

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.

7. ## Re: Prove this by whole cube expansion

thanks man, ibdutt, thanks a lot