>636
(1)　sin(px)*sin(qx) = (1/2){cos((p-q)x) - cos((p+q)x)},　∫[-π,π] cos(nx)dx = 2πδ_(n,0) より、(与式) = πδ_(p,q).
(2)　a_k = (-1)^(k-1) * (2/k).