LOOP IN A TWO PARAMETERS FUNCTION

Hi everybody,

my question is about, how can I programming a loop for a list of values in a 3d function. I'll try to explain myself. I have a 3d surface equation and it works fine with a single values like this:

but I want to repeat the calculation in a loop for a pair of list of values, something like this:

What I'm doing wrong?. Mathcad reports "This variable is undefined. Check that the label is set correctly".

Any idea?

Thanks and congratulations.

Note: equations was obtained from thread "T-stub method" in this forum. Thanks unknown member.