A triangle has three interior angles, and the sum of these angles is always 180°.

This important rule helps us find any missing angle in a triangle when two angles are known.

In this lesson, we will use this rule to calculate the missing angle in different types of triangle problems.

Rule

The sum of the interior angles of a triangle is:

A + B + C = 180°

To find a missing angle, subtract the known angles from 180°.

Example

Find the missing angle x in the triangle:

A = 60°
B = x
C = 30°

60 + x + 30 = 180

90 + x = 180

x = 90°

So the missing angle is 90°.

Video Explanation

Practice

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Continue Learning

1. Types of Triangles
2. Triangles (Basics)
3. Sum of Angles of a Triangle is 180° (Proof)
4. Find the Missing Angle of a Triangle
5. Sum of Exterior Angles of a Triangle (Proof)
6. Exterior Angle Theorem of a Triangle
7. Two Exterior Angles at a Vertex are Equal (Proof)
8. Two Exterior Angles are Equal at a Vertex (Quick Proof)

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