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Polygons can be classified by the number of sides they have.

Number of sides Name of Polygon Figure

3 Triangle

4 Quadrilateral

5 Pentagon

6 Hexagon

7 Heptagon

8 Octagon

9 Nonagon

10 Decagon

n n-gon

Diagonal
A diagonal is a line segment connecting two non-consecutive vertices of a polygon.
( −3)
Number of diagonals of a polygon having n sides=
2
Example: Find the number of diagonals of decagon.
Solution: Decagon has 10 sides, so n = 10.
10(10−3)
Number of diagonals of a polygon having 10 sides =
2
10×7
=
2
2