ANSWER
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EXPLANATION
If ABCD is a parallelogram, then
line AB is parallel to line DC .
This means that,
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and
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are alternating angles.
Alternate angles are congruent.
This implies that,
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We group like terms to obtain,
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This simplifies to,
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Also, if ABCD is a parallelogram then,
BC is parallel to AD. This means that,
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We group like terms to get,
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This simplifies to,
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We divide both side by 4 to get,