Figure 16.15 is an end view of a triangular prism, whose three faces are labeled by the vectors dA1, etc. (The magnitude of dA1 is the area of the corresponding surface and the direction is normal to it. There are two more faces parallel to the plane of the paper, but these do not concern us.) The ends of the three faces form a closed triangle. Explain clearly why this implies that dA1+ dA2+dA3= 0.
Figure 16.15
