6.3: Parallelograms
What if a college wanted to build two walkways through a parallelogramshaped 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.
Watch This
CK12 Foundation: Chapter6ParallelogramsA
Brightstorm: Parallelogram Properties
Guidance
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
 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.
 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.
 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?
 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:
http://www.mathwarehouse.com/geometry/quadrilaterals/parallelograms/interactiveparallelogram.php
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:
Prove:
Statement  Reason 

1. 
Given 
2. 
Definition of a parallelogram 
3. 
Alternate Interior Angles Theorem 
4. 
Reflexive PoC 
5. 
ASA 
6. 
CPCTC 
Example A
Draw a picture. When labeling the vertices, the letters are listed, in order, clockwise.
If
Example B
Find the values of
Opposite sides are congruent, so we can set each pair equal to each other and solve both equations.
Even though
Example C
Show that the diagonals of
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.
Watch this video for help with the Examples above.
CK12 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.
Vocabulary
A parallelogram is a quadrilateral with two pairs of parallel sides.
Guided Practice
1.
2. Find the measures of
3. If
Answers:
1. Because this is a parallelogram, the diagonals bisect each other and
2. Consecutive angles are supplementary so
3.
Practice
 If
m∠S=143∘ in parallelogramPQRS , find the other three angles.  If
AB¯¯¯¯¯¯¯¯⊥BC¯¯¯¯¯¯¯¯ in parallelogramABCD , find the measure of all four angles.  If
m∠F=x∘ in parallelogramEFGH , find expressions for the other three angles in terms ofx .
For questions 411, find the measures of the variable(s). All the figures below are parallelograms.
Use the parallelogram

m∠AWE 
m∠ESV 
m∠WEA 
m∠AVW
In the parallelogram

SO 
NT 
m∠NWS 
m∠SOW
Plot the points
 Find the coordinates of the point at which the diagonals intersect. How did you do this?
 Find the slopes of all four sides. What do you notice?
 Use the distance formula to find the lengths of all four sides. What do you notice?
 Make a conjecture about how you might determine whether a quadrilateral in the coordinate is a parallelogram.
Write a twocolumn proof.
 Opposite Angles Theorem
 Given
 :

ABCD  is a parallelogram with diagonal

BD¯¯¯¯¯¯¯¯  Prove
 :

∠A≅∠C  Parallelogram Diagonals Theorem
Given:
Prove:
Use the diagram below to find the indicated lengths or angle measures for problems 2629. The two quadrilaterals that share a side are parallelograms.

w 
x 
y 
z
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Image Attributions
Here you'll learn what a parallelogram is and how to apply theorems about its sides, angles, and diagonals.