Pythagorean Theorem Lesson for Kids: Definition & Examples

Instructor: Bethany Calderwood

Bethany has taught special education in grades PK-5 and has a master's degree in special education.

An important part of geometry is knowing how to measure shapes. The Pythagorean Theorem is used in the measurement of triangles. In this lesson, you will learn the Pythagorean Theorem and how to use it. Then you will practice on a triangle of your own.


Scientists and mathematicians like to study relationships. They study how a plant or animal relates to its environment, and how numbers relate to nature and music. When a specific relationship is found over and over, they give it a name and share the information. One example is the Pythagorean Theorem, which explains the relationship between the sides of a right triangle.

The Anatomy of a Triangle

Before you can learn about the Pythagorean Theorem, you need to know a few terms.

right triangle

Right Triangle

A right triangle has one ninety-degree angle inside the triangle, which is called a right angle. Mathematicians often mark the right angle with a box. On the diagram of the triangle, the box is drawn in red.


In a right triangle, the hypotenuse is the side directly across from the right angle. It is the only side of the triangle that is not a part of the right angle. On the diagram, the hypotenuse is green.


An exponent appears slightly above and to the right of a number, like this: 32. A number with an exponent is actually a multiplication sentence, and the exponent indicates how many times that number appears in the multiplication sentence. So this example, 32, means 3 x 3. An exponent of 2 is read ''to the second power'' or ''squared.''

The Pythagorean Theorem

The Pythagorean Theorem states the relationship between the sides of a right triangle, when c stands for the hypotenuse and a and b are the sides forming the right angle. The formula is:

a2 + b2 = c2

It is read ''a-squared plus b-squared equals c-squared.''

This picture shows the right triangle with the sides labeled for the Pythagorean Theorem.
pythagorean theorem triangle

Using the Pythagorean Theorem

Leila's school is having a door-decorating contest. Leila's class wants to use ribbon to divide their door into two triangles. They know the length of each side of the door. To find out how long the ribbon needs to be, they can use the Pythagorean Theorem.

Leilas door

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