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# What must one justify to prove that a quadrilateral ABCD is a parallelogram?

Regents Prep explains that the quickest method for proving that a quadrilateral is a parallelogram is if one pair of opposite sides of the quadrilateral are both parallel and congruent. However, several other forms of proof exist as well.

Continue ReadingAccording to Wikipedia, in Euclidean geometry, a parallelogram is a simple quadrilateral, with four edges and four corners, characterized by its two pairs of parallel sides. Because of its parallel orientation, the opposite sides of a parallelogram are equal in length. The opposite angles of a parallelogram are also equal in measure. This congruence is proven by Euclid's parallel postulate, also known as Euclid's fifth postulate; it is part of Euclid's "Elements," which serves as the basis for the shortest method of proof previously described.

The basic definition of a parallelogram lends itself to several other methods of proof. For instance, one can prove that a quadrilateral is a parallelogram if the two pairs of opposite angles in the quadrilateral are congruent. Further, if the consecutive angles in the quadrilateral are supplementary, or equal to 180 degrees, then it is a parallelogram. Finally, one can also prove a quadrilateral is a parallelogram if the two diagonals of the quadrilateral can bisect each other, or divide each other equally in half.

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## What is the perimeter of a parallelogram?

A: The perimeter of a parallelogram, as for any polygon, is the sum of the lengths of its sides. The opposite sides of a parallelogram are parallel and have t... Full Answer >Filed Under: -
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## How do you calculate the area of a parallelogram from vectors?

A: To calculate the area of a parallelogram from vectors, find the cross product of the sides and vectors. The area equals the length of the cross product of ... Full Answer >Filed Under: -
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## What are some proofs related to parallelograms?

A: One proof regarding a parallelogram is used to show that the two pairs of opposite sides are congruent. When given a parallelogram with angles A, B, C and ... Full Answer >Filed Under: -
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## How many sides are there in a parallelogram?

A: A parallelogram is a two-dimensional rhomboid or quadrilateral with four sides. The four sides, or edges, consist of two pairs, each with the opposite side... Full Answer >Filed Under: