In these lesson, we will learn

- about interior angles of triangles
- the properties of the interior angles of a triangle
- how the properties can be used to find the missing angles inside a triangle
- how the sum of interior angles of a triangle can be used to set up equations.

**Related Pages**

Exterior Angles Of A Triangle

Types Of Triangles

Angles In A Triangle

The following diagram shows that the sum of angles in a triangle is 180°. Scroll down the page for more examples and solutions on how to find missing angles in a triangle.

The interior angles of a triangle are the angles **inside** the triangle.

- The sum of the three interior angles in a triangle is always 180°.
- Since the interior angles add up to 180°, every angle must be less than 180°.

**Example:**

Find the value of x in the following triangle.

**Solution:**

x + 24° + 32° = 180° (sum of angles is 180°)

x + 56° = 180°

x = 180° – 56° = 124°

**How to find the missing angle inside a triangle.**

**Example:**

Find the missing angle in a triangle.

A = 15°, B = ?, C = 92°

**How to use the sum of the interior angles of a triangle to solve problems?**

**Example:**

- Write an equation and solve for the unknown.
- Substitute your answer into each expression to determine the measure of the angles.
- Give reasons for your answers.

Using the sum of the interior angles of a triangle, write an equation and solve for the unknown.

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