a=-\frac{A}{2}\cos  \left(\frac{2\pi }{T}t\right)+C_a=-\frac{A}{2}\cos  \left(\frac{2\pi }{T}t\right)+\frac{A}{2}=\frac{A}{2}\left(1-\cos  \left(\frac{2\pi }{T}t\right)\right)