How to Find Missing Angle in a Triangle

Finding a Missing Angle in a Triangle

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?

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°.

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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.

See Also

What is a triangle? What are the types of triangle? What is an angle?