This Instructional Activity is a part of the Lesson 15, Unit 2, Grade 8. Students are introduced to the Pythagorean theorem and are shown an informal proof of the theorem. Students use the Pythagorean theorem to find the length of the hypotenuse of a right triangle. It is crucial that students see the teacher explain several proofs of the Pythagorean theorem and practice using it before being expected to produce a proof on their own.