What is

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A Polygon is a closed figure formed by a line segments.

For basic we should know first the parts of Polygon.

1. The Side or commonly called the Edge of polygon is one of the line segments that make up the polygon.
Adjacent sides it is a pair of sides that share a common endpoint.
2. The Vertices of polygon, this are the endpoints of each side. Adjacent sidesare the endpoints of side.

3. The Diagonal of a polygon is a line segment joining two non-adjacent vertices of the polygon.
4. The Interior angle is the angle formed by two adjacent sides inside the polygon.
5. The Exterior Angle is an angle that is adjacent to and supplementary to an interior angle of the polygon.

A polygon may also defined a UNION of line segments such that:

Each vertex is a common endpoint of two adjacent line segments;

No two adjacent line segments intersects except at an endpoint;

No two segments with the same endpoint are collinear.

to be continued...