Extensión PAR (serie de cosenos con periodo p=2L)

$${\displaystyle f(x)={\frac {a_{0}}{2}}+\sum _{n=1}^{\infty }a_{n}\cos {\frac {n\pi x}{L}}}$$

$$a_{0}={\frac {2}{L}}\int _{-L}^{L}f(x)dx$$

$$a_{n}={\frac {2}{L}}\int _{0}^{L}f(x)\cos {\frac {n\pi x}{L}}dx$$

$$b_{n}=0$$