1. The problem statement, all variables and given/known data This isn't actually a homework question, but I'm revising for my resits and can't do this past paper question. And of course if i don't figure out how to do it it will obviously come up on the exam A plane is tangential to the surface xexp(-x^2 - y^2) - z = 0 at the point where z takes on its maximum value. Find the equation of the plane 2. Relevant equations 3. The attempt at a solution I think I have to find where z is at its maximum and then find the normal to the tangent at that point to enable me to write the plane equation in the form r.n=0 z will be at it's maximum when xexp(-x^2 - y^2) is at its maximum but i don't know when that will be?