Fourier series in the interval (0,2l) Problem 3 | Engineering Mathematics

Find the Fourier series expansion of
f(x) =x^2 in the range of (0,2l). Hence deduce that
1/1^2 +1/2^2 +1/3^2 +⋯=π^2/6ii)1/1^2 -1/2^2 +1/3^2 -…=π^2/12
iii)1/1^2 +1/3^2 +1/5^2 ….=π^2/8