Question

S=cos0 + cos2pi/3 + cos4pi/3 + cos 2pi?

Answer 1

$S=\cos 0+\cos \dfrac{2\pi}{3}+\cos \dfrac{4\pi}{3}+\cos 2\pi\\ \\ =1-\dfrac{1}{2}-\dfrac{1}{2}+1\\ \\ =1$