# Thread: Sum of squares of cosine.

1. ## Sum of squares of cosine.

Find: $cos ^{2}(\frac{\pi }{5})+ cos^{2} (\frac{2\pi }{15})+ cos^{2} (\frac{8\pi }{15})$

2. Originally Posted by arm
Find: $cos ^{2}(\frac{\pi }{5})+ cos^{2} (\frac{2\pi }{15})+ cos^{2} (\frac{8\pi }{15})$
To get help you need to show some effort. Show your work and say where you're stuck.