http://www.ck12.org Chapter 6. Polygons and Quadrilaterals
6.4 Quadrilaterals that are Parallelograms
Here you’ll learn how to prove that a quadrilateral is a parallelogram.
What if four friends, Geo, Trig, Algie, and Calc were marking out a baseball diamond? Geo is standing at home
plate. Trig is 90 feet away at 3rdbase, Algie is 127.3 feet away at 2ndbase, and Calc is 90 feet away at 1stbase. The
angle at home plate is 90◦, from 1stto 3rdis 90◦. Find the length of the other diagonal and determine if the baseball
diamond is a parallelogram. After completing this Concept, you’ll be able to answer questions like this based on
your knowledge of parallelograms.
Watch This
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Click image to the left for more content.CK-12 Foundation: Chapter6QuadrilateralsthatareParallelogramsA
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Click image to the left for more content.KhanAcademy: Opposite Sidesof a Parallelogram Congruent
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Click image to the left for more content.KhanAcademy: Diagonals of aParallelogram Bisect Each Other
Guidance
Recall that a parallelogram is a quadrilateral with two pairs of parallel sides. Even if a quadrilateral is not marked
with having two pairs of sides, it still might be a parallelogram. The following is a list of theorems that will help you
decide if a quadrilateral is a parallelogram or not.
Opposite Sides Theorem Converse: If the opposite sides of a quadrilateral are congruent, then the figure is a
parallelogram.