The projections of the surface ∑ 1 on the xOy and yOz planes are all line segments, so the integral of dxdy dydz is 0, or the feature y=3 and dy=0 directly brought into the surface. The projection of ∑ 1 on the xOz plane is a circle. On the other hand, the angle between the surface and the Y axis is acute, so the double integral of the projection is multiplied by+1 (if it is obtuse angle multiplied by-1).