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Parallelograms

Quadrilateral with two pairs of parallel sides.

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Parallelograms

What if you were told that is a parallelogram and you are given the length of and the measure of ? What can you determine about , , , and ? After completing this Concept, you'll be able to apply parallelogram theorems to answer such questions.

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CK-12 Parallelograms

Guidance

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

Notice that each pair of sides is marked parallel (for the last two shapes, remember that when two lines are perpendicular to the same line then they are parallel). Parallelograms have a lot of interesting properties. (You can explore the properties of a parallelogram at: http://www.mathwarehouse.com/geometry/quadrilaterals/parallelograms/interactive-parallelogram.php)

Facts about Parallelograms

1) Opposite Sides Theorem: If a quadrilateral is a parallelogram, then both pairs of opposite sides are congruent.

If then

2) Opposite Angles Theorem: If a quadrilateral is a parallelogram, then both pairs of opposite angles are congruent.

If then

3) Consecutive Angles Theorem: If a quadrilateral is a parallelogram, then all pairs of consecutive angles are supplementary.

If then

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

If then

Example A

is a parallelogram. If , find the measure of the other angles.

First draw a picture. When labeling the vertices, the letters are listed, in order.

If , then by the Opposite Angles Theorem.

Example B

Find the values of and .

Remember that opposite sides of a parallelogram are congruent. Set up equations and solve.

Example C

Prove the Opposite Sides Theorem.

Given: is a parallelogram with diagonal

Prove:

Statement Reason
1. is a parallelogram with diagonal 1. Given
2. 2. Definition of a parallelogram
3. 3. Alternate Interior Angles Theorem
4. 4. Reflexive PoC
5. 5. ASA
6. 6. CPCTC

The proof of the Opposite Angles Theorem is almost identical. You will try this proof in the problem set.

CK-12 Parallelograms

Guided Practice

1. Show that the diagonals of bisect each other.

2. is a parallelogram, and . Solve for .

3. Find the measures of and in the parallelogram below:

Answers:

1. Find the midpoint of each diagonal.

Because they are the same point, the diagonals intersect at each other’s midpoint. This means they bisect each other.

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

3. Consecutive angles are supplementary so which means that . and are alternate interior angles and since the lines are parallel (since its a parallelogram), that means that .

Practice

is a parallelogram. Fill in the blanks below.

  1. If , then ______.
  2. If , then ______.
  3. If = ______.
  4. If = ______.
  5. If = ______.
  6. If , then = ______.
  7. If in parallelogram , find the other three angles.
  8. If in parallelogram , find the other three angles.
  9. If in parallelogram , find the measure of all four angles.
  10. If in parallelogram , find the other three angles.

For questions 11-18, find the values of the variable(s). All the figures below are parallelograms.

Use the parallelogram to find:

Find the point of intersection of the diagonals to see if is a parallelogram.

Fill in the blanks in the proofs below.

  1. Opposite Angles Theorem

Given: is a parallelogram with diagonal

Prove:

Statement Reason
1. 1. Given
2. 2.
3. 3. Alternate Interior Angles Theorem
4. 4. Reflexive PoC
5. 5.
6. 6.
  1. Parallelogram Diagonals Theorem

Given: is a parallelogram with diagonals and

Prove:

Statement Reason
1. 1.
2. 2. Definition of a parallelogram
3. 3. Alternate Interior Angles Theorem
4. 4.
5. 5.
6. 6.
  1. Find and . (The two quadrilaterals with the same side are parallelograms.)

Vocabulary

parallelogram

parallelogram

A quadrilateral with two pairs of parallel sides. A parallelogram may be a rectangle, a rhombus, or a square, but need not be any of the three.

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