# The Interior Angles of a Triangle

# The Interior Angles of a Triangle

A triangle has 3 interior angles.The interior angles of a triangle determine the type of triangle.

The interior angles of a triangle add up to 180°.

# Interior Angles and the Type of Triangles

Triangles are**equilateral**,

**isosceles**or

**scalene**depending on how many of the interior angles are equal to each other.

Triangles are

**acute**,

**obtuse**or

**right**depending on how whether all angles are less than 90°, 1 angle is more than 90° or 1 angle is equal to 90°.

Read more about the types of triangles

# The Interior Angles of a Triangle Add Up to 180°

The interior angles of any triangle add up to 180°.# A Real Example of How the Interior Angles of a Triangle Add Up to 180°

The interior angles of the triangle below add up to 180°.50° + 60° + 70° = 180°.

# How to Find a Missing Interior Angle of a Triangle

Because all the interior angles of a triangle add up to 180°, it is possible to find a missing interior angle when the others are known.**Question:**What is the missing angle, x, in the triangle below?

**Step 1:**Add up the interior angles of the triangle and make this equal to 180°.

75° + 50° + x = 180°.

**Step 2:**Rearrange the equation to find x.

x = 180° - 75° - 50° = 55°.

Read more about how to find a missing angle in a triangle

# More on the Interior Angles of Triangles

The slider below shows more about the interior angles of a triangle:##### Curriculum

##### Interactive Test

**show**

##### Note

**WHAT IS A TRIANGLE?**

A triangle is a 2-dimensional shape with three sides and three angles.

# HOW TO FIND THE MISSING ANGLE OF A TRIANGLE

The interior angles of a triangle always add up to 180°.where A, B and C are the interior angles of a triangle.

A + B + C = 180°

is an algebraic equation.

When 2 of the 3 angles are known, the 3

^{rd}angle can be found by substituting the known values into the equation and then rearranging the equation.

Consider the triangle below:

Find C.

A + B + C = 180°

We know A = 45, B = 55.

45° + 55° + C = 180°

This is an algebraic equation that contains addition.

The 45° and 55° are being added to C.

Subtract them from both sides to make C the subject of the equation.

C = 180° - 45° - 55°

C = 80°