Chapter 7.7, Problem 12P

Show that if a real $f(x)$ is expanded in a complex exponential Fourier series ${\displaystyle \underset{-\infty }{\overset{\infty }{\sum }}{c}_n}{e}^{inx},$ then $c_{-n}={\bar{c}}_n,$ where ${\bar{c}}_n$ means the complex conjugate of $c_n$.