Hello people im new to this site and have a problem i cant seem to solve:
Use the method of eigenfunction expansions to solve
$y''(x) + y(x) = \cos x$ $∀x ∈ (0, π)$ subject to $\left\{ \begin{array}{c l} y(0)=0 \\ y(\pi)=0 \end{array}\right.$
Ive been stuck on this for quite a while so any help will be appreciated