Theory:

A parallelogram is a flat shape with four sides in which opposite sides parallel and equal in length.
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Where \(|\) and \(||\) denotes equal sides.
 
Properties of parallelogram:
  • Opposite sides are parallel.
  • Opposite sides are equal in length.
  • Opposite angles are equal. (Angles \(A\) and \(D\) are same and angles \(B\) and \(C\) are same.)
  • Angles "\(A\)" and "\(C\)"  or "\(B\)" and "\(D\)" add up to \(180°\) so that they are supplementary angles. (For example, Two angles (\(140°\) and \(40°\)) are supplementary angles because they add up to \(180°\)).
  • Adjacent angles are supplementary in parallelogram.
Important!
For example: A parallelogram became a rectangle where all angles are right angles.
 
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