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A 4sided polygon is a quadrilateral
Polygon
Polygon is a simple closed figure made of line segments
A quadrilateral has
 4 vertices − A, B, C, D
 4 sides − AB, BC, CD, DA
 4 angles − ∠A, ∠B, ∠C, ∠D
Opposite sides
 AB & CD are opposite sides

AD & BC are opposite sides
Adjacent sides
 AB, BC are adjacent sides
 AB, AD are adjacent sides
 BC, CD are adjacent sides

CD, DA are adjacent sides"
Opposite angles
 ∠A, ∠C are opposite angles
 ∠B, ∠D are opposite angles
Adjacent angles
 ∠A, ∠B are adjacent angles.
 ∠A, ∠D are adjacent angles.
 ∠B, ∠C are adjacent angles.
 ∠C, ∠D are adjacent angles.
Diagonals of a quadrilateral
Line joining two opposite vertices of a quadrilateral is a diagonal.
Here, AC, BD are diagonals of the quadrilateral.
And they meet at point O.