# Informal Proof of the Pythagorean Theorem

Video solutions to help Grade 8 students learn the Pythagorean theorem and an informal proof of the theorem.

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Lessons for Grade 8
Common Core For Grade 8
## New York State Common Core Math Module 2, Grade 8, Lesson 15

### Lesson 15 Student Outcomes

• Students will know the Pythagorean theorem and be shown an informal proof of the theorem.

• Students will use the Pythagorean theorem to find the length of the hypotenuse of a right triangle.

Lesson 15 Summary
Given a right triangle ABC with being the vertex of the right angle, then the sides AC and BC are called the legs
of △ ABC and AB is called the hypotenuse of △ ABC.
Take note of the fact that side a is opposite the angle A, side b is opposite the angle B, and side c is opposite the
angle C.
The Pythagorean theorem states that for any right triangle, a

^{2} + b

^{2} = c

^{2}.

Classwork
Concept Development

The Pythagorean theorem is a famous theorem that will be used throughout much of high school mathematics. For that
reason, you will see several proofs of the theorem throughout the year and have plenty of practice using it. The first
thing you need to know about the Pythagorean theorem is what it states:

Pythagorean theorem: If the lengths of the legs of a right triangle are a and b, and the length of the hypotenuse is c,
then a

^{2} + b

^{2} = c

^{2}.

Given a right triangle ABC with C being the vertex of the right angle, then the sides AC and BC are called the legs of
△ ABC and AB is called the hypotenuse of △ ABC.

Discussion

The first proof of the Pythagorean theorem that you will see requires you to know some basic facts about geometry:

1. Congruent triangles have equal areas.

2. Triangles can be considered congruent using the Side-Angle-Side Criterion (SAS).

3. The triangle sum theorem. (∠ sum of △)

a. All corresponding parts of congruent triangles are congruent.

a. For right triangles, the two angles that are not the right angle have a sum of 90°. (∠ sum of △)

Example 1

Now that we know what the Pythagorean theorem is, let’s practice using it to find the length of a hypotenuse of a right
triangle.

Determine the length of the hypotenuse of the right triangle.

Example 2

Determine the length of the hypotenuse of the right triangle.

Exercises 1–5

For each of the exercises, determine the length of the hypotenuse of the right triangle shown. Note: Figures not drawn
to scale.