## Geometry: Common Core (15th Edition)

The formula for calculating the sum of the interior angles of a regular polygon is: (n - 2) × 180° where n is the number of sides of the polygon. As a regular octagon has 8 sides, we plug 8 into this equation: (8 - 2) × 180° = 6 × 180° = 1080° This tells us that all of the interior angles of a regular octagon add up to 1080°. However, we need to know the measure of just one angle. To find this, we can simply divide 1080° by 8, as there are 8 interior angles in an octagon. 1080 $\div$ 8 = 135°