# Thread: Sum of cosines.

1. ## Sum of cosines.

Prove:$\displaystyle cos (\frac{\pi }{7})- cos (\frac{2\pi }{7})+ cos (\frac{3\pi }{7})=\frac{1}{2}$

2. Originally Posted by arm
Prove:$\displaystyle cos (\frac{\pi }{7})- cos (\frac{2\pi }{7})+ cos (\frac{3\pi }{7})=\frac{1}{2}$
Please make some effort. Show your work and say where you're stuck.