A nonagon or enneagon is a polygon with 9 sides and 9 vertices.
A regular nonagon is a polygon with 9 equal sides and angles.
Angles of the Nonagon
The sum of interior angles of a nonagon = (9 − 2) · 180° = 1,260°
The value of an interior angle of a regular nonagon is: 1,260º : 9 = 140º
The central angle of a regular nonagon measures: 360º : 9 = 40º
Diagonals of the Nonagon
The number of diagonals in a regular nonagon = 9 · (9 − 3) : 2 = 27
Perimeter of a Regular Nonagon
Perimeter = 9 · l
Area of a Regular Nonagon