Past exam of the mathematical methods II course of the University of Oxford / 2016 / 10 / b / ii / Solution

The plane cuts the positive coordinate axes at $(3,0,0)$, $(0,3,0)$ and $(0,0,3/2)$. Choose its unit normal vector $(1,1,2)/\sqrt{6}$, pointing away from the origin. In the parametrization $(x,3-x-2z,z)$, the vector surface element for this orientation is $(1,1,2)dxdz$. Thusand the triangular projection givesFor the opposite orientation the answer is $-9/4$. The normal direction must be stated because this open surface has no prescribed orientation in the question.