Students will learn how to find the diagonal length of a rectangle, given the length of each side. They must already know the Pythagorean theorem. Here's a video demonstration of this lesson. Give your students problems involving both squares and rectangles. Students should notice the diagonal length is irrational, except when the side and diagonal lengths form a Pythagorean triple. Next, give your students problem #6 from this page. A solution can be found here. Finally, give your students this challenge.