Find the Fourier series solution to the differential equation x"+x=t It's given that x(0)=x(1)=0 So, I'm trying to find a Fourier serie to x(t) and f(t)=t, and I'm know it must a serie of sin... So here's my question...the limits of integration to the Bn, how do I define them? Will it be like 0 to L to both series? And about the x", after a I find the f(x) Fourier series I must just derive it and replace in the x"??