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| Theorem 3 |
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| A quadrilateral is a parallelogram if one pair of opposite sides are equal and parallel. |
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| ABCD is a quadrilateral in which AB||CD and AB=CD. |
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| ABCD is a parallelogram. |
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| Join AC. |
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| In triangles ABC and ADC, |
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| AB=CD (given) |
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| AC = AC (common side) |
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 (corresponding parts of corresponding triangles) |
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| Since these are alternate angles, AD||BC. |
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| Thus in the quadrilateral ABCD, AB||CD and AD||BC. |
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 ABCD is a parallelogram. |
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