# Interior Angles of Polygons

### An Interior Angle is an angle inside a shape Another example: ## Triangles

The Interior Angles of a Triangle add up to 180°

Let’s try a triangle: 90° + 60° + 30° = 180°

It works for this triangle

Now tilt a line by 10°: 80° + 70° + 30° = 180°

It still works!
One angle went up by 10°,
and the other went down by 10°

(A Quadrilateral has 4 straight sides)

Let’s try a square: 90° + 90° + 90° + 90° = 360°

A Square adds up to 360°

Now tilt a line by 10°: 80° + 100° + 90° + 90° = 360°

It still adds up to 360°

### Because there are 2 triangles in a square … The interior angles in a triangle add up to 180°

… and for the square they add up to 360°

… because the square can be made from two triangles!

## Pentagon A pentagon has 5 sides, and can be made from three triangles, so you know what …

… its interior angles add up to 3 × 180° = 540°

And when it is regular (all angles the same), then each angle is 540° / 5 = 108°

(Exercise: make sure each triangle here adds up to 180°, and check that the pentagon’s interior angles add up to 540°)

The Interior Angles of a Pentagon add up to 540°

## The General Rule

So the general rule is:

Sum of Interior Angles = (n-2) × 180°

Each Angle (of a Regular Polygon) = (n-2) × 180° / n

Perhaps an example will help:

### Example: What about a Regular Decagon (10 sides) ? Sum of Interior Angles = (n-2) × 180°

= (10-2)×180° = 8×180° = 1440°

And it is a Regular Decagon so:

Each interior angle = 1440°/10 = 144°

Note: Interior Angles are sometimes called “Internal Angles”

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