Internal Angles of Regular Polygon

Theorem
The internal angles of a regular polygon is given by the formula $A = \frac {180^\circ(n-2)} {n}$, where $n$ is the number of sides of the polygon.

Proof
From Sum of Internal Angles of Polygon, we have that the sum of all internal angles of a $n$-gon (a polygon with $n$ sides) is $180^\circ(n-2)$.

From the definition of a regular polygon, we have that all the internal angles of a polygon are all equivalent.

Therefore, the size of each internal angle in a regular polygon with $n$ sides is $\frac {180^\circ(n-2)} {n}$.