I need some advise on curve fitting:
I have a 3 input function f(x,y,z). The function value should be non-negative.
I have some raw data from experiments (both input and output) based on which I'm trying to use Excel solver to find some coefficients of 2 order function of following format f(x,y,z)= (ax+by+cz)^2 +dx+ey+fz.
For the specific values of the experiment, the function is fine with the fitted data. However for data which is not as same as in experiments, I can receive negative values of the function.
Can you please suggest me what to do?