If the mass per unit area of a surface is given by ρ=xy, find the mass if S is the part of the cylinder x2+z2=25 which is in the first octant and contained within the cylinder x2+y2=16. So here was my attempt. I parametrized the curve. x2+z2=25 r(u, v) = <5cos(u), v, 5sin(u)> I then plugged into the bounds x2+y2=16. 25cos2(u) + v2 = 16 v = sqrt(16 - 25cos2(u)) Next I took the cross product of ru X rv. Its magnitude is a constant 5. Now I solved the integral with bounds 0 < u < Pi/2 and 0 < v < sqrt(16 - 25cos2(u)) ∫∫25 v cos(u) du dv This returns the result of -25/3 but since we are looking for a mass I submitted the answer of positive 25/3 and this is wrong. I checked the integration on my calculator and it gets the same result.