## The Lesson

To find the missing angle of a triangle, use the fact that the interior angles of a triangle add up to 180°. If any of the angles (A, B or C) is unknown, it can be found as long as the other two angles are known.

## How to Find the Missing Angle in a Triangle

Finding the missing angle in a triangle is easy.

## Question

What is the missing angle A in the triangle below? # 1

The angles add up to 180°.
A + 65° + 75° = 180°

# 2

Find A by subtracting the known angles from 180°.
A = 180° − 65° − 75° = 40°

The missing angle A is 40°.

## Lesson Slides

The slider below shows another real example of how to find the missing angle in a triangle. Open the slider in a new tab

## How to Find the Missing Angle in an Equilateral Triangle

In the example above, we found the missing angle in a scalene triangle: a triangle where all three angles are different. It is even easier to find the missing angle in an equilateral triangle: where all three angles are equal to each other and all three sides are the same length. All three angles in an equilateral triangle are 60°. ## How to Find the Missing Angle in an Isosceles Triangle

An isosceles triangle has two angles that are equal to each other.

## An Equation to Find the Missing Angle

The interior angles of a triangle always add up to 180°. In this formula, A, B and C are the interior angles of a triangle. The formula...
A + B + C = 180°
... is an algebraic equation. A, B or C can be made the subject of the equation by subtracting the other two letters from both sides:
A = 180° − B − C B = 180° − A − C C = 180° − A − B
To find the missing angle: • Label each angle in the triangle with a letter.
• The unknown letter will be to the left of the =. In the image above, A is the unknown angle. Choose the equation with A to the left of the =.
A = 180° − B − C
• Insert the values of the known letters into the right hand side of the equation. In the image above, B = 64° and C = 72°.
A = 180° − 64° − 72°
• Calculate the unknown angle.
A = 44°