Prove that in the power series ∑ n = 0 ∞ c n x n {\displaystyle {}\sum _{n=0}^{\infty }c_{n}x^{n}} of the hyperbolic cosine the coefficients c n {\displaystyle {}c_{n}} are 0 {\displaystyle {}0} if n {\displaystyle {}n} is odd.
