Quadrilateral with Congruent Opposite Sides are Parallelogram

This is the first of a series of post about the conditions of a quadrilateral to be a parallelogram.  In this post, I will be discussing the proof that if the opposite sides of a quadrilateral are congruent, then it is a parallelogram. Given Quadrilateral  
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The Parallel Line Postulate

In the post on Five Postulates, the fifth postulates states that If two lines are drawn which intersect a third in such a way that the sum of the inner angles on one side is less than two right angles, then the two lines inevitably must intersect each other on that side if extended far enough. This is […]
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