Interior angles in regular polygons
In a regular polygon, all angles and sides are the same size. Once we can work out the total sum of interior angles in a polygon, it is fairly straightforward to be able to find the size of one angle.
For example, the interior angle sum of a pentagon is 540°. A pentagon has 5 angles, and in a regular pentagon, each of these angles will be the same size. So we divide 540° by 5 to work out that each angle must be 108°.
We must remember that this only works for regular polygons - in irregular shapes, the angles could be different sizes.