This is question that came up in one of the past papers I have been doing for my exams. Its says that if $\omega=\cos(\pi/5)+i\sin(\pi/5)$. What is $\omega^3-\omega^2$. I can find $\omega^3$ and $\omega^2$ by De Moivre's Theorem. But I cant make much headway into how to subtract these? 
The answer they give is $2cos(\dfrac{3\pi}{5})$

Please, some help will be greatly appreciated. I cant seem to find help over the internet either.