A regular octadecagon (an eighteen sided polygon) would have nine diagonals.
A five-sided polygon is called a pentagon. A regular five-sided polygon is simply called a regular pentagon
A sixteen-sided regular polygon is called a hexadecagon or a hexakaidecagon.
A one thousand sided regular polygon is called a chiliagon.chiliagon (a polygon with 1000 sides)
A regular fifty-sided polygon is called a pentacontagon.
A 1000-sided regular polygon is called a chiliagon.
A 55-sided regular polygon would be called a pentacontakaipentagon.
A square is a regular four-sided polygon.
Expressed using systematic polygon names, an 83-sided regular polygon would be called an octacontakaitrigon.
A nineteen-sided regular polygon is called an enneadecagon.
a regular polygon
quadrilateral