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| Sufficient conditions for a quadrilateral to be a parallelogram |
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| We can state the defining property of a parallelogram as follows: |
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| "If a quadrilateral is a parallelogram, then its opposite sides are equal". |
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| Converse |
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| "If both pairs of opposite sides of a quadrilateral are equal, then the quadrilateral is a parallelogram". |
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| The converse statement stated above is a necessary condition for a quadrilateral to be a parallelogram. Similarly, we may formulate the following two other conditions for a quadrilateral to be a parallelogram. |
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 "If the diagonals of a quadrilateral bisect each other, the quadrilateral is a parallelogram". |
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 "If either pair of opposite sides of a quadrilateral are equal and parallel, the quadrilateral is a parallelogram". |
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