In these lessons, we will learn

- the Triangle Sum Theorem (with worksheets)
- how to prove the Triangle Sum Theorem
- how to solve problems using the Triangle Sum Theorem

**Related Pages**

Angles In A Triangle

More Geometry Lessons

Geometry Worksheets

The following diagram shows the Triangle Sum Theorem. Scroll down the page for examples and solutions using the Triangle Sum Theorem.

The Triangle Sum Theorem states that

The sum of the three interior angles in a triangle is always 180°.

The Triangle Sum Theorem is also called the Triangle Angle Sum Theorem or Angle Sum Theorem.

**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°

Worksheet 1,

Worksheet 2 using Triangle Sum Theorem

**How to prove that the sum of the angles of a triangle is 180 degrees (Triangle Sum Theorem)?**

This is a two-column proof that involves parallel lines and the alternate angle theorem.

**How to prove the Triangle Sum Theorem using right triangles?**

We will start with right triangles, and then expand our proof later to include all triangles.

The following videos show more examples of how to solve problems related to the triangle sum theorem using Algebra.

**How to Find the Missing Angle in a Triangle Using the Triangle Sum Theorem?**

Step 1. Write out the equation by adding all the angles and making them equal to 180°

Step 2. Solve for x.

Step 3: Substitute to find the missing angles.

**Use the triangle sum theorem to find the base angle measures given the vertex angle in an isosceles triangle**

**Solve for angles using the Triangle Sum Theorem**

**Example:**

Using the diagram shown, find the value of x and the measure of each missing angle in the triangle.

**Triangle Properties**

**Example:**

Find the values of x and y for a given triangle problem. This problem uses the Triangle Sum Theorem and the Corollary to the Triangle Sum Theorem.

**Triangle Properties**

**Example:**

Find the values of x and y for a given triangle problem. This problem involves linear pair angles as well as the Triangle Sum Theorem.

**Solve for angles using the Triangle Sum Theorem and ratios**

**Example:**

The measures of the angles of a triangle are in the ratio 2:5:8. Find the measure of each angle.

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