Subtracting roots of unity. Specifically $\omega^3 - \omega^2$ 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.

$$\omega=\cos(\pi/5)+i\sin(\pi/5)$$
$$\omega^3-\omega^2=\cos(3\pi/5)+i\sin(3\pi/5)-(\cos(2\pi/5)+i\sin(2\pi/5))$$
$$\omega^3-\omega^2=\cos(3\pi/5)-\cos(2\pi/5)+i(\sin(3\pi/5)-\sin(2\pi/5))$$
$$\omega^3-\omega^2=\cos(3\pi/5)-\cos(2\pi/5)+i(\sin(3\pi/5)-\sin(2\pi/5))$$
$$\omega^3-\omega^2=-2\sin(\pi/2)\cdot\sin(\pi/10)-2i\cos(\pi/2)\cdot\sin(\pi/10)$$
$$\omega^3-\omega^2=-2\sin(\pi/10)$$
$$\omega^3-\omega^2=-2\cdot\dfrac{\sqrt{5}-1}{4}$$
$$\omega^3-\omega^2=\dfrac{1-\sqrt{5}}{2}$$
use some trig. formula:
$\cos C-\cos D=2\cdot \sin\dfrac{C+D}{2}\cdot \sin \dfrac{D-C}{2}$
and $\sin C-\sin D=2\cdot \cos\dfrac{C+D}{2}\cdot \sin \dfrac{C-D}{2}$
and $\sin (\pi/10)=\dfrac{\sqrt5-1}{4}$
