p.519 #33
This demonstrates the fact that, for a trapezoid , given a point in its interior, it is not necessarily true that:
Drag point around and notice that the sum of the areas of the two triangles is not always half of the area of the entire trapezoid. Since all four triangles sum to the area of the trapezoid, this means that the two sums of areas of the triangles are equal if and only if each sum is half of the area of the trapezoid.