1. The problem statement, all variables and given/known data[/b] For the surface with parametric equations r(s,t) = <st, s+t, s-t>, find the equation of the tangent plane at (2,3,1). Find the surface area under the restriction s^2 + t^2 <= 1. 2. Relevant equations[/b] 3. The attempt at a solution[/b] I already found that the equation of the plane is -2x+3y-z=4. I just don't know what to do with that parametrization to find the surface area. I tried plugging in the values of r(s,t) into x, y, and z, then integrating after converting s and t (I tried s = cos(theta) and t = sin(theta)) to polar, but that didn't work. Any ideas on how to get started?