Lesson 9
The Converse
Lesson Narrative
This lesson guides students through a proof of the converse of the Pythagorean Theorem. Then students have an opportunity to decide if a triangle with three given side lengths is or is not a right triangle.
Learning Goals
Teacher Facing
 Determine whether a triangle with given side lengths is a right triangle using the converse of the Pythagorean Theorem.
 Generalize (orally) that if the side lengths of a triangle satisfy the equation $a^2+b^2=c^2$ then the triangle must be a right triangle.
 Justify (orally) that a triangle with side lengths 3, 4, and 5 must be a right triangle.
Student Facing
Let’s figure out if a triangle is a right triangle.
Learning Targets
Student Facing
 I can explain why it is true that if the side lengths of a triangle satisfy the equation $a^2+b^2=c^2$ then it must be a right triangle.
 If I know the side lengths of a triangle, I can determine if it is a right triangle or not.
Glossary Entries

Pythagorean Theorem
The Pythagorean Theorem describes the relationship between the side lengths of right triangles.
The diagram shows a right triangle with squares built on each side. If we add the areas of the two small squares, we get the area of the larger square.
The square of the hypotenuse is equal to the sum of the squares of the legs. This is written as \(a^2+b^2=c^2\).

hypotenuse
The hypotenuse is the side of a right triangle that is opposite the right angle. It is the longest side of a right triangle.
Here are some right triangles. Each hypotenuse is labeled.

legs
The legs of a right triangle are the sides that make the right angle.
Here are some right triangles. Each leg is labeled.