A octadecagon is a polygon with 18 sides and 18 vertices.
A regular octadecagon is a polygon with 18 equal sides and angles.
Angles of the Octadecagon
The sum of interior angles of an octadecagon = (18 − 2) · 180° = 2,880°
The value of an interior angle of a regular octadecagon is: 2,880º : 18 = 160º
The central angle of a regular octadecagon measures: 360º : 18 = 20º
Diagonals of the Octadecagon
The number of diagonals in a regular octadecagon = 18 · (18 − 3) : 2 = 135
Perimeter of a Regular Octadecagon
Perimeter = 18 · l
Area of a Regular Octadecagon