Encontrando los coeficientes

$f(x)={\frac {A_{0}}{2}}+\sum _{m=1}^{\infty }A_{m}cos(mkx)+\sum _{m=1}^{\infty }B_{m}sen(mkx)\qquad (1)$

donde:

$A_{0}={\frac {2}{\lambda }}\int \limits _{0}^{\lambda }f(x)dx\qquad \color {Black}(2)$

$A_{m}={\frac {2}{\lambda }}\int \limits _{0}^{\lambda }f(x)cos(mkx)dx\qquad \color {Black}(3)$ $B_{m}={\frac {2}{\lambda }}\int \limits _{0}^{\lambda }f(x)sen(mkx)dx\qquad \color {Black}(4)$