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