problem with integrating a sum



I'm trying solve Laplace's equation with some boundary conditions and I
have problem with integration a sum:

declare(n,integer);

declare(m,integer);

E1:'integrate(sin((%pi*m*y)/a)*sum(C(n)*%e^(-(%pi*n*x)/a)*sin((%pi*n*y)/a),n,1,N),y)='integrate(sin((%pi*m*y)/a),y)*V0

ev(E1,nouns) - nothing happend on the left side of equation

why integrate on left can't cope with sum? intosum() doesn't help. How
to move 'integrate inside the summation?

RT