The area of the square is 82 D 64, whereas the rectangle, which seems to have the same
constituent parts, has an area 5 Ð 13 D 65, and so the area has apparently been increased
by one square unit. The puzzle is easy to explain. The points a, b, c, and d do not all lie
on the diagonal of the rectangle, but instead are the vertices of a parallelogram whose
area is exactly equal to the extra unit of area