Power Series Representation

cosine

$$\cos (x)=\sum _{k=0}^{\infty }(-1{)}^k\frac{1}{(2k)!}{x}^{2k}$$

sine

$$\sin (x)=\sum _{k=0}^{\infty }(-1{)}^k\frac{1}{(2k+1)!}{x}^{2k+1}$$

B) Related

C) References