## The Lesson

The sum of the interior angles of a polygon is found using the formula: In this formula, n is the number of sides of the polygon. The formula tells you what the interior angles of a polygon add up to.

## How to Find the Sum of the Interior Angles of a Polygon

Finding the sum of the interior angle of a polygon is easy.

## Question

What is the sum of the interior angles of a pentagon? # 1

Sum of interior angles = (n − 2) × 180°

# 2

Substitute the number of sides into the formula. A pentagon has 5 sides. In our example, n = 5.
Sum of interior angles = (5 − 2) × 180° Sum of interior angles = 3 × 180° Sum of interior angles = 540°

The sum of the interior angles of a pentagon is 540°.

## Lesson Slides

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## What Are the Interior Angles of a Polygon?

The interior angles of a polygon are the angles between two sides, inside the shape.

## Why Does the Formula Work?

Any polygon can be split into a number of triangles, each of which has interior angles adding up to 180°. The number of triangles is 2 less than the number of sides and there are there are 180° of interior angles for every triangle:
(n − 2) × 180°
This is why the formula works.