Sorry forgot to give the definition of mfx(x,y,ufx):
 

mfx:=(x,y,ufx)->sum(ufx[k]*(x-x[k])*(y-y[k]), k);

I think I may have used the wrong word. I meant f(x,y) to be an unknown function. The only thing I know is that f(x,y) is a function of x and y, which are then functions of r and theta.