Triangles Teaching Resources

Are you teaching about triangles in your math classes this school year? Need triangle lesson plans, worksheets and digital activities?

The teachers of Teach Starter have got you covered with printables and digital activities aligned with both TEKS and the Common Core math curriculum.

Help your students learn to identify right triangles, decompose polygons into triangles and plenty more with editable resources you can easily adjust to differentiate instruction.

Whether it's your first year teaching about triangles, or you're just looking for some handy tips and tricks, the Teach Starter teacher team has you covered. Read on for a kid-friendly definition of triangles, a look at the different types of these fundamental shapes and more!

What Is a Triangle? A Kid-Friendly Definition

Triangles are among the core shapes students learn about, and you've likely known what a triangle is for a long, long time. But do you have a simple definition you can use to explain these common shapes to your students?

That's where we come in!

A triangle is a shape that has just three sides. Triangles are polygons, which means they are two-dimensional, and they have straight sides.

All triangles have three angles created at the points where their sides meet.

Triangles can be found all around us. You may notice them on buildings, bridges and many other structures.

What Is a Right Triangle?

As students begin to identify parallel and perpendicular lines and learn about the angles that are created when lines cross, it's time to talk about right triangles!

This type of triangle is unique from all other types in that it has one angle that measures 90 degrees.

What Is a Hypotenuse? This Right Triangle Feature Explained

A right triangle has a special side called a hypotenuse. This is the side of the triangle that is opposite of the right angle.

What Is the Pythagorean Theorem?

The hypotenuse is special because it can be used to help us find the length of the other sides of a right triangle.

To do this, we use a mathematical rule called the Pythagorean theorem.

The theorem tells us that the square of the length of the hypotenuse of a right triangle equals the sum of the squares of the lengths of the two other sides.

This equation is represented as a² + b² = c².

In this equation, c is the length of the hypotenuse. A and b represent the lengths of the other two sides.

What Is an Equilateral Triangle?

Another common type of triangle is the equilateral triangle.

An equilateral triangle is a 2D shape that has three equal sides. Each angle in an equilateral triangle is also equal, and the angles always measure 60 degrees.

What Is an Isosceles Triangle?

An isosceles triangle is a triangle that has two equal sides. The two angles that are opposite the equal sides are also equal.