The Interior Angles of a Triangle
(KS2, Year 6)

homesitemapgeometrythe interior angles of a triangle
A triangle has 3 interior angles.

interior angles triangle explained 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.

interior angles equilateral isosceles scalene 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°.

interior angles acute obtuse right the types of triangles

The Interior Angles of a Triangle Add Up to 180°

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

interior angles of a triangle The interior angles of the triangle below add up to 180°. interior_angles_add_up_to_180_degrees
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 two other interior angles are known. In the triangle below, we know two interior angles and want to find the third, θ. interior_angles_triangle_missing_angle The three interior angles must sum to 180°:
θ + 50° + 70° = 180°
By subtracting the two known angles from 180°, we find the missing angle:
θ = 180° − 50° − 70° = 60°
how to find a missing angle in a triangle

Lesson Slides

The slider below shows more about the interior angles of a triangle.

What Is a Triangle?

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

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This page was written by Stephen Clarke.

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