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It may not be understood at first glance, but all squares, rectangles, and rhombi are parallelograms. Therefore, all the properties of parallelograms apply to these quadrilaterals as well! In this lesson, theorems about parallelograms will be discussed to understand this concept better.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.


Explore

Investigating Properties of Parallelograms

In the applet below, a parallelogram, a rectangle, a rhombus, and a square can be selected and rotated clockwise about the point of intersection of its diagonals. What can be noted when these polygons are rotated
quadrilaterals
Note that all the quadrilaterals in the applet are parallelograms. Additionally, when rotated each quadrilateral is mapped onto itself. Considering this information, what conclusions can be made about the opposite sides of a parallelogram?
Discussion

Properties of a Parallelogram's Opposite Sides

A conclusion that can be made from the previous exploration is that the opposite sides of a parallelogram are congruent. This is explained in detail in the following theorem.

Rule

Parallelogram Opposite Sides Theorem

The opposite sides of a parallelogram are congruent.

parallelogram

In respects to the characteristics of the diagram, the following statement holds true.

Proof

Using Congruent Triangles

This theorem can also be proven by using congruent triangles. Consider the parallelogram and its diagonal

parallelogram and one of its diagonals
It can be noted that two triangles are formed with as a common side.
By the definition of a parallelogram, and are parallel. Therefore, by the Alternate Interior Angles Theorem, it can be stated that and that Furthermore, by the Reflexive Property of Congruence, is congruent to itself.
parallelogram and one of its diagonals and the two pair of congruent angles
Consequently, and have two pairs of congruent angles and an included congruent side.
Therefore, by the Angle-Side-Angle Congruence Theorem, and are congruent triangles.
Since corresponding parts of congruent figures are congruent, is congruent to and is congruent to

Furthermore, it can be stated whether a quadrilateral is a parallelogram just by checking if its opposite sides are congruent.

Rule

Converse Parallelogram Opposite Sides Theorem

If the opposite sides of a quadrilateral are congruent, then the polygon is a parallelogram.

parallelogram

Following the above diagram, the statement below holds true.

If and then is a parallelogram.

Proof

This theorem can be proven by using congruent triangles. Consider the quadrilateral whose opposite sides are congruent, and its diagonal By the Reflexive Property of Congruence, this diagonal is congruent to itself.

parallelogram and one of its diagonals
Therefore, by the Side-Side-Side Congruence Theorem, and are congruent triangles.
Since corresponding parts of congruent figures are congruent, corresponding angles of and are congruent.
parallelogram and one of its diagonals

Finally, by the Converse of the Alternate Interior Angles Theorem, is parallel to and is parallel to Therefore, by the definition of a parallelogram, is a parallelogram.

parallelogram

This proves the theorem.

If and then is a parallelogram.

Another conclusion that can be made from the exploration is that the opposite angles of a parallelogram are congruent. This is explained in detail in the following theorem.
Rule

Parallelogram Opposite Angles Theorem

In a parallelogram, the opposite angles are congruent.

For the parallelogram the following statement holds true.

Proof

This theorem can be proved by using congruent triangles. Consider the parallelogram and its diagonal

Opposite sides of a parallelogram are parallel. Therefore, by the Alternate Interior Angles Theorem it can be stated that and Furthermore, by the Reflexive Property of Congruence, is congruent to itself.

Two angles of and their included side are congruent to two angles of and their included side. By the Angle-Side-Angle Congruence Theorem, and are congruent triangles.
Since corresponding parts of congruent figures are congruent, and are congruent angles.

By drawing the diagonal and using a similar procedure, it can be shown that and are also congruent angles.

Furthermore, it can be determined whether a quadrilateral is a parallelogram just by looking at its opposite angles.

Rule

Converse Parallelogram Opposite Angles Theorem

If both pairs of opposite angles of a quadrilateral are congruent, then the quadrilateral is a parallelogram.

A parallelogram with congruent opposite angles

Based on the above diagram, the following statement holds true.

Proof

Assume that is a quadrilateral with opposite congruent angles. It should be noted that congruent angles have the same measure. Then, let be the measure of and and be the measure of and

By the Polygon Interior Angles Theorem, the sum of the interior angles of a quadrilateral is With this information, a relation can be found between the consecutive interior angles of
Simplify
Since the consecutive interior angles of are supplementary. Therefore, by the Converse Consecutive Interior Angles Theorem, it can be concluded that opposite sides of are parallel.
A polygon ABCD with one movable vertex
Consequently, by the definition of a parallelogram, is a parallelogram.

Completed Proof

Proof:
Proof of the theorem
Example

Solving Problems Using Properties of a Parallelogram

To be able to be carefree and enjoy a soccer match over the weekend, Vincenzo wants to complete his Geometry homework immediately after school. He is given a diagram showing a parallelogram, and asked to find the values of and

A parallelogram with its horizontal sides labeled 2x+5 and 5x-10. The four interior angles, starting from the top left corner and proceeding clockwise, are labeled 5a+10, 3b-10, a-10, and 10b+60.
Find the values of and to help Vincenzo be carefree for the match!

Hint

In a parallelogram, opposite sides are congruent and opposite angles are congruent.

Solution

First, for simplicity, the value of will be found. After that, the values of and will be calculated.

Value of

According to the Parallelogram Opposite Sides Theorem, the opposite sides of a parallelogram are congruent. Therefore, the opposite sides have the same length. With this information, an equation in terms of can be expressed.
This equation can now be solved for
Solve for

Values of and

According to the Parallelogram Opposite Angles Theorem, the opposite angles of a parallelogram are congruent. That means the opposite angles have the same measure. Knowing this, a system of equations can be expressed.
This system will be solved by using the Substitution Method. For simplicity, the degree symbol will be removed.
Solve by substitution
Explore

Investigating the Diagonals of a Parallelogram

In the following applet, a parallelogram is shown. By dragging one of its vertices, different types of parallelograms such as squares, rectangles, and rhombi can be formed. By using the measuring tool provided, investigate what relationships exist between the diagonals of each parallelogram.
parallelogram
After investigating each parallelogram type, what relationships between the diagonals of the parallelograms were discovered?
Discussion

Properties of a Parallelogram's Diagonals

A conclusion that can be made from the previous exploration is that the diagonals of a parallelogram intersect at their midpoint. This is explained in detail in the following theorem.

Rule

Parallelogram Diagonals Theorem

In a parallelogram, the diagonals bisect each other.

If is a parallelogram, then the following statement holds true.


Proof

Using Congruent Triangles

This theorem can be proven by using congruent triangles. Consider the parallelogram and its diagonals and Let be the point intersection of the diagonals.

Since and are parallel, by the Alternate Interior Angles Theorem it can be stated that and that Furthermore, by the Parallelogram Opposite Sides Theorem it can be said that

Here, two angles of and their included side are congruent to two angles of and their included side. Therefore, by the Angle-Side-Angle Congruence Theorem and are congruent triangles.
Since corresponding parts of congruent triangles are congruent, is congruent to and is congruent to

By the definition of a segment bisector, both segments and are bisected at point Therefore, it has been proven that the diagonals of a parallelogram bisect each other.

Also, a quadrilateral can be identified as a parallelogram just by looking at its diagonals.

Rule

Converse Parallelogram Diagonals Theorem

If the diagonals of a quadrilateral bisect each other, then the quadrilateral is a parallelogram.

Quadrilateral with diagonals that bisect each other

Based on the diagram above, the following relation holds true.

If and bisect each other, then is a parallelogram.

Proof

Let be point of intersection of the diagonals of a quadrilateral. Since the diagonals bisect each other, is the midpoint of each diagonal.

Quadrilateral with diagonals that bisect each other

Because and are vertical angles, they are congruent by the Vertical Angles Theorem. Therefore, by the Side-Angle-Side Congruence Theorem, and are congruent triangles. Since corresponding parts of congruent figures are congruent, and are congruent.

Quadrilateral with diagonals that bisect each other

Applying a similar reasoning, it can be concluded that and are congruent triangles. Consequently, and are also congruent.

Quadrilateral with diagonals that bisect each other

Finally, since both pairs of opposite sides of quadrilateral are congruent, the Converse Parallelogram Opposite Sides Theorem states that is a parallelogram.

Quadrilateral with diagonals that bisect each other
Example

Solving Problems Using Properties of a Parallelogram's Diagonals

Vincenzo has one last exercise to finish before going to a soccer match. He has been given a diagram showing a parallelogram. He is asked to find the value of and

A parallelogram with its longer diagonal bisected by two segments labeled 9 and 2x+1, and its shorter diagonal bisected by two segments labeled y+4 and 3y.
Find the values of and and help Vincenzo finish his homework!

Hint

The diagonals of a parallelogram bisect each other.

Solution

According to the Parallelogram Diagonals Theorem, the diagonals of a parallelogram bisect each other.

parallelogram
With this information, two equations can be written.
These can be solved one at a time. Equation (I) will be solved first.
Solve for
The value of is Finally, Equation (II) will be solved.
Solve for
The value of is
Discussion

Investigating Rotations of Parallelograms

Consider a rigid motion that rotates a parallelogram about the point of intersection of its diagonals.
parallelogram
Since a rotation is a rigid motion, the preimage and the image are congruent figures. Furthermore, because corresponding parts of congruent figures are congruent, the three statements below hold true.
  • Opposite sides of a parallelogram are congruent.
  • Opposite angles of a parallelogram are congruent.
  • The diagonals of a parallelogram bisect each other.
Note that the Parallelogram Opposite Sides Theorem, the Parallelogram Opposite Angles Theorem, and the Parallelogram Diagonals Theorem have been proved using a rotation about the point of intersection of the diagonals.
Discussion

Diagonals of a Rectangle

It can be determined whether a parallelogram is a rectangle just by looking at its diagonals. Furthermore, if a parallelogram is a rectangle, a statement about its diagonals can be made.

Rule

Rectangle Diagonals Theorem

A parallelogram is a rectangle if and only if its diagonals are congruent.

rectangle with its diagonals marked

Based on the diagram, the following relation holds true.

is a rectangle

Two proofs will be provided for this theorem. Each proof will consist of two parts.

  • Part I: If is a rectangle, then
  • Part II: If then is a rectangle.

Proof

Using Similar Triangles

This proof will use similar triangles to prove the theorem.

Part I: Is a Rectangle

Suppose is a rectangle and and are its diagonals. By the Parallelogram Opposite Sides Theorem, the opposite sides of a parallelogram are congruent. Therefore, and are congruent. Additionally, by the Reflexive Property of Congruence, or is congruent to itself.

rectangle with its diagonals marked
Since the angles of a rectangle are right angles, by the definition of congruent angles, . Consequently, and have two pairs of congruent sides and congruent included angles.
Therefore, by the Side-Angle-Side Congruence Theorem, the triangles are congruent.
Because corresponding parts of congruent triangles are congruent, and which are the diagonals of are congruent.

Part II: Is a Rectangle

Consider the parallelogram and its diagonals and such that

rectangle with its diagonals marked

By the Parallelogram Opposite Sides Theorem, Additionally, by the Reflexive Property of Congruence, is congruent to itself.

rectangle with its diagonals marked
The sides of are congruent to the sides of
Therefore, by the Side-Side-Side Congruence Theorem, Moreover, since corresponding parts of congruent triangles are congruent, is congruent to
Note that and are consecutive angles. By the Parallelogram Consecutive Angles Theorem, these angles are supplementary. With this information, it can be concluded that both and are right angles.
Additionally, by the Parallelogram Opposite Angles Theorem, and Because all of the angles are right angles, is a rectangle.

Proof

Using Transformations

This proof will use transformations to prove the theorem.

Part I: Is a Rectangle

Consider the rectangle and its diagonals and Let be the point of intersection of the diagonals.

rectangle with its diagonals marked

Let and be the midpoints of and Then, a line through and the midpoints and can be drawn.

rectangle with its diagonals marked
Note that and are congruent segments. Because congruent segments have the same length, the distance between and equals the distance between and Therefore, is the image of after a reflection across Similarly, is the image of after the same reflection.
rectangle with its diagonals marked
Since lies on a reflection across maps onto itself.
Reflection Across
Preimage Image
The table shows that the images of the vertices of are the vertices of Therefore, is the image of after a reflection across Since a reflection is a rigid motion, this proves that the triangles are congruent.
rectangle with its diagonals marked
Because corresponding parts of congruent figures are congruent, and Additionally, by the Parallelogram Diagonals Theorem, the diagonals of the rectangle bisect each other. Therefore, all four segments are congruent.
rectangle with its diagonals marked
Each diagonal of the parallelogram consists of the same two congruent segments. By the Segment Addition Postulate, the diagonals are congruent.

Part II: Is a Rectangle

Consider the parallelogram and its diagonals and such that By the Parallelogram Diagonals Theorem, the diagonals of a rectangle bisect each other at

rectangle with its diagonals marked

By the Parallelogram Opposite Sides Theorem, and

rectangle with its diagonals marked

Let and be the midpoints of and Then, a line through and the midpoints and can be drawn.

rectangle with its diagonals marked
As shown before, and are the respective images of and after a reflection across Therefore, since is the image of after a reflection across the triangles are congruent.
rectangle with its diagonals marked
Let and be the midpoints of of and By following the same reasoning, is the image of after a reflection across Therefore, the triangles are congruent.
rectangle with its diagonals marked
The parallelogram consists of four triangles in which the opposite triangles are congruent. Therefore, the corresponding angles of these triangles are congruent. Additionally, because all triangles are isosceles, the angles opposite congruent sides are congruent as well.
rectangle with its diagonals marked
Each angle of the parallelogram is the sum of the same two congruent angles. Therefore, all angles of the parallelogram are congruent.
Moreover, by the Parallelogram Consecutive Angles Theorem, and are supplementary. With this information, it can be concluded that both angles are right triangles.
Because all of the angles are congruent, the angles of the parallelogram are right triangles. Therefore, is a rectangle.
Example

Solving Problems Using the Diagonals of a Rectangle

Zosia arrives early to a Harry Styles concert! She notices something about the stage, so she uses a napkin as paper and draws a diagram. The stage is a rectangle that she labels as

A rectangle with vertices labeled A, B, C, and D in clockwise order, starting from the top left corner. Diagonal AC is labeled 4x+3, and diagonal BC is labeled 45-2x.
Find the lengths of its diagonals to help Zosia understand the stage that she will see her favorite artist sing on!

Hint

In a rectangle, the diagonals are congruent.

Solution

By the Rectangle Diagonals Theorem, the diagonals of a rectangle are congruent. This means that and have the same length. With this information, an equation in terms of can be written.
The above equation will be solved for
Solve for
The value of was found. Next, this value can be substituted in any of the expressions for the length of a diagonal. In this case, will be arbitrarily substituted in the expression for
Evaluate right-hand side
It was found that the length of is Since the diagonals of a rectangle are congruent, the length of is also
Discussion

Diagonals of a Rhombus

As with rectangles, it can also be determined whether a parallelogram is a rhombus just by looking at its diagonals.

Rule

Rhombus Diagonals Theorem

A parallelogram is a rhombus if and only if its diagonals are perpendicular.

Parallelogram with diagonals drawn

Based on the diagram, the following relation holds true.

Parallelogram is a