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Quadrilaterals that are Parallelograms

Ways to show if a quadrilateral has two pairs of parallel sides.

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Quadrilaterals that are Parallelograms

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 base, Algie is 127.3 feet away at base, and Calc is 90 feet away at base. The angle at home plate is , from to is . 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

CK-12 Foundation: Chapter6QuadrilateralsthatareParallelogramsA

Khan Academy: Opposite Sides of a Parallelogram Congruent

Khan Academy: Diagonals of a Parallelogram 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.

Opposite Angles Theorem Converse: If the opposite angles of a quadrilateral are congruent, then the figure is a parallelogram.

Parallelogram Diagonals Theorem Converse: If the diagonals of a quadrilateral bisect each other, then the figure is a parallelogram.

Theorem: If a quadrilateral has one set of parallel lines that are also congruent, then it is a parallelogram.

Each of these theorems can be a way to show that a quadrilateral is a parallelogram.

Proof of the Opposite Sides Theorem Converse:

Given:

Prove: is a parallelogram

Statement Reason
1. Given
2. Reflexive PoC
3. SSS
4. CPCTC
5. Alternate Interior Angles Converse
6. is a parallelogram Definition of a parallelogram

To show that a quadrilateral is a parallelogram in the plane, you will need to use a combination of the slope formulas, the distance formula and the midpoint formula. For example, to use the Definition of a Parallelogram, you would need to find the slope of all four sides to see if the opposite sides are parallel. To use the Opposite Sides Converse, you would have to find the length (using the distance formula) of each side to see if the opposite sides are congruent. To use the Parallelogram Diagonals Converse, you would need to use the midpoint formula for each diagonal to see if the midpoint is the same for both. Finally, you can use the last Theorem in this Concept (that if one pair of opposite sides is both congruent and parallel then the quadrilateral is a parallelogram) in the coordinate plane. To use this theorem, you would need to show that one pair of opposite sides has the same slope (slope formula) and the same length (distance formula).

Example A

Write a two-column proof.

Given: and

Prove: is a parallelogram

Statement Reason
1. and Given
2. Alternate Interior Angles
3. Reflexive PoC
4. SAS
5. CPCTC
6. is a parallelogram Opposite Sides Converse

Example B

Is quadrilateral a parallelogram? How do you know?

For part a, the opposite angles are equal, so by the Opposite Angles Theorem Converse, is a parallelogram. In part b, the diagonals do not bisect each other, so is not a parallelogram.

Example C

Is the quadrilateral a parallelogram?

First, find the length of and .

, so if the two lines have the same slope, is a parallelogram.

Slope Slope

Therefore, is a parallelogram.

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter6QuadrilateralsthatareParallelogramsB

Concept Problem Revisited

First, we can use the Pythagorean Theorem to find the length of the second diagonal.

This means that the diagonals are equal. If the diagonals are equal, the other two sides of the diamond are also 90 feet. Therefore, the baseball diamond is a parallelogram.

Guided Practice

1. What value of would make a parallelogram?

2. Is the quadrilateral a parallelogram?

3. If a quadrilateral has one pair of parallel sides is it a parallelogram?

Answers:

1. from the markings. Therefore, would be a parallelogram if as well.

In order for to be a parallelogram, must equal 7.

2. Let’s use the Parallelogram Diagonals Converse to determine if is a parallelogram. Find the midpoint of each diagonal.

Midpoint of

Midpoint of

Because the midpoint is not the same, is not a parallelogram.

3. Although it has one pair of parallel sides, this quadrilateral is not a parallelogram because its opposite sides are not necessarily congruent.

Interactive Practice

Explore More

For questions 1-11, determine if the quadrilaterals are parallelograms. If they are, write a reason.

For questions 12-14, determine the value of and that would make the quadrilateral a parallelogram.

For questions 15-17, determine if is a parallelogram.

Write a two-column proof.

  1. Parallelogram Diagonals Theorem Converse Given: Prove: is a parallelogram
  2. Given: Prove: is a parallelogram

Suppose that and are three of four vertices of a parallelogram.

  1. Depending on where you choose to put point , the name of the parallelogram you draw will change. Sketch a picture to show all possible parallelograms. How many can you draw?
  2. If you know the parallelogram is named , what is the slope of side parallel to ?
  3. Again, assuming the parallelogram is named , what is the length of ?

The points and are the vertices of quadrilateral . Plot the points on graph paper to complete problems 23-26.

  1. Find the midpoints of sides and . Label them and respectively.
  2. Connect the midpoints to form quadrilateral . What does this quadrilateral appear to be?
  3. Use slopes to verify your answer to problem 24.
  4. Use midpoints to verify your answer to problem 24.

Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 6.4. 

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