The name of a 21 sided polygon is Icosihenagon.
The name for this polygon is a nonagon,
The name of a 21 sided polygon is Icosihenagon.
The name of a 7 sided polygon is Heptagon.
icosikaihenagon
A hendecagon or an undecagon is the name of a polygon which has eleven sides.
Apothem!
It will have 21 sides or a 21-gon
Not surprisingly, it is called a side!
"Heptagon" is the name of a 7-sided polygon.
If measured on the inside of the polygon, the it is an interior angle.
A five-sided polygon is called a "pentagon."