It's quite acceptable to call it a 21-agon but what's more important is knowing how to work out its properties
The name of a 21 sided polygon is Icosihenagon.
The name of a 21 sided polygon is Icosihenagon.
icosihenagon= 21-sided polygon icosihenagon= 21-sided polygon
An icosikaihena is a polygon with 21 sides
a triple heptagon.
The name of a 12 sided polygon is Dodecagon.
name of 4000 sided polygon
A 8 sided polygon is called a octagon.
Though many polygons have a specific name not all do. It is accepted practice to use the number of sides as the name if one is not assigned (or known). A 21 sided polygon does not have a specific name so could quite reasonably be call a 21-ogon.
The term for a 21 sided polygon is an icosikaihenagon. Icosi is the Greek for the numerical 20 and henagon means 1.
The name of a 10 sided polygon is Decagon.
The name of a 9 sided polygon is Nonagon.