Stationary point is that the fisrt derivative of g is equal to 0 at (0,0). I interpolated f in this triangle and took the integral to see what is going to happen and for interpolation function, I have used(just see in one article and esed the same interpolation function)

and after integration I saw that Maple failed to interpolate the first term of K since (0,0) is stationary point (at stationary points integrating is not possible.)
Now I want to integrate f in this triangle but without using (0,0) but enough close points to (0,0). The problem is that although I tried several times I couldn't get any function like K in 3 dimension. How I can get such kinda function with using interpolation?