the name of a 20 sided 3d shape is called a icosahedron
An icosagon is the name of the 20 sided polygon.
Any of these shapes is practically circular. A 20-sided shape is an icosagon. a 50-sided shape is a pentacontagon (aka pentecontagon). A 100-sided shape is a hectogon (aka hecatontagon). A 1000-sided shape is a chiliagon.
A 5 sided shape is a pentagon. Its perimeter has no relevance.
Icosagon
icosahedron
The name for a 57 sided shape is derived by combining the name for a 50 sided shape with the name for a 7 sided shape. A 57 sided shape is called a pentacontaheptagon.
A 20-sided polyhedron is an icosahedron. A 20-sided polygon is an icosagon.
Icosahedron is a 20 sided polygon.
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The name of a tweleve sided shape is a dodecagon.
Icosahedron is the shape, but it's just called 20 sided if you want to buy one.