urista

I have a problem on my homework that is really confusing. I need to solve the partial differential equation in a spherical shell with inner radius = a and outer radius=b: (Laplacian u)=1 in spherical coordinates. The boundary conditions are u=0 on the inner radius r=a, and du/dr=0 on outer radius r=b. The part that is really confusing is that the (Laplacian u)=1 not zero which doesn't make since to me physically. I can't use seperation of variables because the PDE is inhomogeneous. I am really confused, would someone help me please?