What must one justify to prove that a quadrilateral ABCD is a parallelogram?

Answer

In order to prove that a quadrilateral is a parallelogram one must be able to justify that both of the pairs of the opposite sides are in fact parallel with each other. However it's also important to note that if only one pair of the opposite sides are congruent and parallel the quadrilateral is still considered to be a parallelogram. It can sometimes be rather tricky and confusing when it comes to proving quadrilaterals are parallelograms, that's why it's important to review the many different aspects of proving this.
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