How do I plot a 3d surface over a specified domain?

I apologise if this has been answered before but I cannot find the answer!

I want to plot the surface: 3a(b-a)^2 / 24b over the 'wedge' in the positive orthant with a \leq b.

Answer 1

[Walter Roberson]

Create a rectangular grid that is the bounding box. For points inside the wedge, set the grid value to the expression you gave. For points outside the wedge, set the grid value to NaN. At the end, surf() the result.