help for poisson equation

tommewang

I used finite element method to solve poisson equation--d/dx(du/dx)+d/dy(du/dy)=f(x,y) in a square domain. f(x,y)=200(only at the mid point of the domain)and u(x,y)=0.at all boundary. I have examine my stiffness matrix and stress matrix,but with my own solver I can not get the symmetric answers.I wonder if there are any one can help me with the solver (no matter iterative method or direct ones). thank alot.