1. The problem statement, all variables and given/known data A triangle is defined by the 3 points P=(1,0,0), Q=(0,2,0) and R=(0,0,2). Set up the double integral over its area. 3. The attempt at a solution The triangle can be described as a plane 2x+y+z+2=0, with xE [0,1], yE [0,2], zE [0,2]. I parametrized it into r(u,v) =u*PQ+v*PR+P = u[-1,2,0]+v[-1,0,2]+(1,0,0). But what's the domain of r(u,v)? I think it is uE [0,1-v], vE[0,1], but even if it is I found it using only gut feeling.