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6.3: Parallelograms

Difficulty Level: At Grade Created by: CK-12
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What if a college wanted to build two walkways through a parallelogram-shaped courtyard between two buildings? The walkways would be 50 feet and 68 feet long and would be built on the diagonals of the parallelogram with a fountain where they intersect. Where would the fountain be? After completing this Concept, you'll be able to answer questions like this by applying your knowledge of parallelograms.

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CK-12 Foundation: Chapter6ParallelogramsA

Brightstorm: Parallelogram Properties


A parallelogram is a quadrilateral with two pairs of parallel sides. Here are some examples:

Notice that each pair of sides is marked parallel. As is the case with the rectangle and square, recall that two lines are parallel when they are perpendicular to the same line. Once we know that a quadrilateral is a parallelogram, we can discover some additional properties.

Investigation: Properties of Parallelograms

Tools Needed: Paper, pencil, ruler, protractor

  1. Draw a set of parallel lines by placing your ruler on the paper and drawing a line on either side of it. Make your lines 3 inches long.
  2. Rotate the ruler and repeat this so that you have a parallelogram. Your second set of parallel lines can be any length. If you have colored pencils, outline the parallelogram in another color.
  3. Measure the four interior angles of the parallelogram as well as the length of each side. Can you conclude anything about parallelograms, other than opposite sides are parallel?
  4. Draw the diagonals. Measure each and then measure the lengths from the point of intersection to each vertex.

To continue to explore the properties of a parallelogram, see the website:


In the above investigation, we drew a parallelogram. From this investigation we can conclude:

Opposite Sides Theorem: If a quadrilateral is a parallelogram, then the opposite sides are congruent.

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

Consecutive Angles Theorem: If a quadrilateral is a parallelogram, then the consecutive angles are supplementary.

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

To prove the first three theorems, one of the diagonals must be added to the figure and then the two triangles can be proved congruent.

Proof of Opposite Sides Theorem:

Given: ABCD is a parallelogram with diagonal BD¯¯¯¯¯¯¯¯

Prove: AB¯¯¯¯¯¯¯¯DC¯¯¯¯¯¯¯¯, AD¯¯¯¯¯¯¯¯BC¯¯¯¯¯¯¯¯

Statement Reason
1. ABCD is a parallelogram with diagonal BD¯¯¯¯¯¯¯¯ Given
2. AB¯¯¯¯¯¯¯¯ || DC¯¯¯¯¯¯¯¯, AD¯¯¯¯¯¯¯¯ || BC¯¯¯¯¯¯¯¯ Definition of a parallelogram
3. ABDBDC, ADBDBC Alternate Interior Angles Theorem
4. DB¯¯¯¯¯¯¯¯DB¯¯¯¯¯¯¯¯ Reflexive PoC
6. AB¯¯¯¯¯¯¯¯DC¯¯¯¯¯¯¯¯, AD¯¯¯¯¯¯¯¯BC¯¯¯¯¯¯¯¯ CPCTC

Example A

ABCD is a parallelogram. If mA=56, find the measure of the other three angles.

Draw a picture. When labeling the vertices, the letters are listed, in order, clockwise.

If mA=56, then mC=56 because they are opposite angles. B and D are consecutive angles with A, so they are both supplementary to A. mA+mB=180,56+mB=180,mB=124. mD=124.

Example B

Find the values of x and y.

Opposite sides are congruent, so we can set each pair equal to each other and solve both equations.

6x74xx=2x+9=16=4y2+3=12  y2=9   y=3 or 3

Even though y=3 or -3, lengths cannot be negative, so y=3.

Example C

Show that the diagonals of FGHJ bisect each other.

The easiest way to show this is to find the midpoint of each diagonal. If it is the same point, you know they intersect at each other’s midpoint and, by definition, cuts a line in half.

Midpoint of FH¯¯¯¯¯¯¯¯: (4+62, 542)=(1,0.5)Midpoint of GJ¯¯¯¯¯¯¯: (312, 322)=(1,0.5)

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter6ParallelogramsB

Concept Problem Revisited

By the Parallelogram Diagonals Theorem, the fountain is going to be 34 feet from either endpoint on the 68 foot diagonal and 25 feet from either endpoint on the 50 foot diagonal.


A parallelogram is a quadrilateral with two pairs of parallel sides.

Guided Practice

1. SAND is a parallelogram, SY=4x11 and YN=x+10. Solve for x.

2. Find the measures of a and b in the parallelogram below:

3. If mB=72 in parallelogram ABCD, find the other three angles.


1. Because this is a parallelogram, the diagonals bisect each other and SYYN.


2. Consecutive angles are supplementary so 127+mb=180 which means that mb=53. a and b are alternate interior angles and since the lines are parallel (since its a parallelogram), that means that ma=mb=53.

3. mD=72 as well, because opposite angles are congruent. A and C are supplementary with D, so mA=mC=108.


  1. If mS=143 in parallelogram PQRS, find the other three angles.
  2. If AB¯¯¯¯¯¯¯¯BC¯¯¯¯¯¯¯¯ in parallelogram ABCD, find the measure of all four angles.
  3. If mF=x in parallelogram EFGH, find expressions for the other three angles in terms of x.

For questions 4-11, find the measures of the variable(s). All the figures below are parallelograms.

Use the parallelogram WAVE to find:

  1. mAWE
  2. mESV
  3. mWEA
  4. mAVW

In the parallelogram SNOW,ST=6,NW=4,mOSW=36,mSNW=58 and mNTS=80. (diagram is not drawn to scale)

  1. SO
  2. NT
  3. mNWS
  4. mSOW

Plot the points E(1,3),F(3,4),G(5,1),H(1,2) and use parallelogram EFGH for problems 20-23.

  1. Find the coordinates of the point at which the diagonals intersect. How did you do this?
  2. Find the slopes of all four sides. What do you notice?
  3. Use the distance formula to find the lengths of all four sides. What do you notice?
  4. Make a conjecture about how you might determine whether a quadrilateral in the coordinate is a parallelogram.

Write a two-column proof.

  1. Opposite Angles Theorem
  2. Given
  3. :
  4. ABCD
  5. is a parallelogram with diagonal
  6. BD¯¯¯¯¯¯¯¯
  7. Prove
  8. :
  9. AC
  10. Parallelogram Diagonals Theorem

Given: ABCD is a parallelogram with diagonals BD¯¯¯¯¯¯¯¯ and AC¯¯¯¯¯¯¯¯

Prove: AE¯¯¯¯¯¯¯¯EC¯¯¯¯¯¯¯¯, DE¯¯¯¯¯¯¯¯EB¯¯¯¯¯¯¯¯

Use the diagram below to find the indicated lengths or angle measures for problems 26-29. The two quadrilaterals that share a side are parallelograms.

  1. w
  2. x
  3. y
  4. z

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    Jul 17, 2012
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