Chapter 14.3, Problem 15P

Here is an old-fashioned way to make a rectangular foundation for a house. Take a pair of identical pieces of wood for the length of the house and another pair of identical pieces of wood for the width of the house. Place the wood on the ground to show approximately where the foundation will go. The pieces of wood now form a quadrilateral whose opposite sides are the same length. Measure the two diagonals of the quadrilateral, and keep adjusting the quadrilateral until the two diagonals are the same length. Explain why the quadrilateral must now be a rectangle. In other words, explain why the quadrilateral must have 4 right angles