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?## Step-by-Step:

## 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°

## Answer:

The missing angle A is 40°.## 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

- 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°

## You might also like...

geometryunderstanding the interior angles of a trianglefinding a missing angle in an isosceles triangleunderstanding angles

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