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Proving that a Quadrilateral is a Parallelogram  Lesson 56
Today, we really only covered the following things...again, the key is to know all the properties on the Venn Diagram we did yesterday...these all fall from those and the definition of parallelograms. So, the ways to prove a quadrilateral is a parallelogram are:
You should note that one of the properties on the Venn Diagram is missing as a way of proving that a quadrilateral is a parallelogram...the one about the consecutive angles being supplementary. The problem with that one is that it would take a long time to prove four pairs of angles supplementary...therefore, we don't use it very often, if at all.
Here's an example using the third of these properties:
