How can you sketch the graph of the Fourier series, the Fourier Cosine Series and Fourier Sine series for f(x)=x^2

How can one find the Fourier series of f(x)=x^3 -(pi^2)*x based on knowing the cosine Fourier series for f(x)=x? I first did it using the x series in sines and integrating it twice, and that comes out easily, but can't understand how to do it starting with the cosines version of f(x)=x, which is given as x =pi/2 + sum [(2[-1^n)-1] cos(nx)/(pi*n^2)]