Find minimum or maximum of multi-variable function

[jte]

One thing that can be done is to graph the zero sets of the two partial derivatives and see where they overlap. I'm attaching a screen shot of what comes up in the Advanced Graphing app if you do that.

   

That there are three colours is, unfortunately, not as obvious as might be desired.

If you switch the tracing mode to intersections between the two sets (via Trace / PoI / Intersections with...), it's fairly clear that you can trace along the line segment between (14,55) and (23,95).

(...although Trace / Inside can give nicer coordinates near the line segment endpoints.) Using a long hold to position the cursor also shows a difference in the overlap region, in that you must choose which set to use with the tracing cursor.