A_n=\frac{1}{\pi}\int^{\frac{-2\pi}{3}}_{-\pi}\left(-\frac{\pi}{3}\right)\cos{nx}dx+\frac{1}{\pi}\int^{\frac{\pi}{3}}_{-\frac{\pi}{3}}\frac{\pi}{3}\cos{nx}dx+\frac{1}{\pi}\int^{\pi}_{\frac{2\pi}{3}}\left(-\frac{\pi}{3}\right)\cos{nx}dx