Polygons: How Many Sides?

A geometrical figure with three or more sides is called a polygon or a polyhedron. Here are the names for some polygons.

3 triangle, trigon
4 quadrilateral, tetragon
5 pentagon
6 hexagon
7 heptagon
8 octagon
9 nonagon, enneagon
10 decagon
11 hendecagon
12 dodecagon, duodecagon
13 triskaidecagon, tridecagon
14 tetrakaidecagon, tetradecagon
15 pentadecagon
16 hexadecagon
17 heptadecagon
18 octadecagon
19 enneadecagon
20 icosagon

How Many Angles?

A polygon has as many angles as it has sides. For example, a triangle has 3 sides and 3 angles. A pentagon has 5 sides and 5 angles. An octadecagon has 18 sides and 18 angles!

Finding Perimeter and Circumference Numbers and Formulas Decimal Equivalents of Common Fractions