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**Proof of parallelogram theorem "If 2 pairs of opposite angles congruent, then par..."**

I was just wondering, why exactly does "If 2 pairs of opposite angles congruent" prove that a quadrilateral is a parallelogram? Does it have something to do with the fact that the sum of the interiors equals 360, so 2x+2y=360? I like knowing why the theorems work, so if anyone knows the proof for this I would love to see it.