8_4__Rhombuses_-_Rectangles_and_Squares 5-MINUTE CHECK
8-4 RHOMBUSES, RECTANGLES AND SQUARES
Students will analyze &amp; identify Rhombusses,
Rectangles &amp; Squares by their properties.
 Why? So you can solve carpentry problems, as
seen in EX 4.
 Mastery is 80% or better on 5-minute checks
and practice problems.

PROPERTIES OF SPECIAL
PARALLELOGRAMS – SKILL DEVELOPMENT

In this lesson, you will study three special
types of parallelograms: rhombuses,
rectangles and squares.
A rectangle is a parallelogram
with four right angles.
A rhombus is a parallelogram
with four congruent sides
A square is a parallelogram with
four congruent sides and four right
angles.
VENN DIAGRAM SHOWS RELATIONSHIPS-- MEMORIZE
 Each shape has the properties of every group that it
belongs to. For instance, a square is a rectangle, a
rhombus and a parallelogram; so it has all of the
properties of those shapes.
Properties of
specific
parallelograms
rhombuses
rectangles
squares
Rhombuses
Rectangles
EX. 1: DESCRIBING A SPECIAL PARALLELOGRAM

Decide whether the statement is always, sometimes, or never
true.
a. A rhombus is a rectangle.
b. A parallelogram is a rectangle.
parallelograms
rhombuses
rectangles
squares
Rhombuses
Rectangles
EX. 1: DESCRIBING A SPECIAL PARALLELOGRAM
Decide whether the statement is always, sometimes, or never true.
a.
A rhombus is a rectangle.
The statement is sometimes true. In the Venn diagram, the regions for
rhombuses and rectangles overlap. IF the rhombus is a square, it is a
rectangle.

parallelograms
rhombuses
rectangles
squares
Rhombuses
Rectangles
EX. 1: DESCRIBING A SPECIAL PARALLELOGRAM
Decide whether the statement is always, sometimes, or never true.
b.
A parallelogram is a rectangle.
The statement is sometimes true. Some parallelograms are rectangles. In
the Venn diagram, you can see that some of the shapes in the
parallelogram box are in the area for rectangles, but many aren’t.

parallelograms
rhombuses
rectangles
squares
Rhombuses
Rectangles
THEOREMS TO KNOW
8.11 A parallelogram is a rhombus if and only if
its diagonals are perpendicular.
 8.12 A parallelogram is a rhombus if and only if
each diagonal bisects a pair of opposite angles.
 8.13 A parallelogram is a rectangle if and only
if its diagonals are congruent.

EX. 2: USING PROPERTIES OF SPECIAL
PARALLELOGRAMS

ABCD is a rectangle. What
else do you know about ABCD?

Because ABCD is a rectangle, it
has four right angles by
definition. The definition also
states that rectangles are
parallelograms, so ABCD has all
the properties of a
parallelogram:



Opposite sides are parallel and
congruent.
Opposite angles are congruent
and consecutive angles are
supplementary.
Diagonals bisect each other.
EXAMPLE 1
For any rhombus QRST, decide whether the statement
is always or sometimes true. Draw a sketch and
a.
Q
S
SOLUTION
a.
By definition, a rhombus is a
parallelogram with four
congruent sides. By Theorem
8.4, opposite angles of a
parallelogram are congruent.
Q
So,
S .The statement is
always true.
TAKE NOTE:


A rectangle is defined as a parallelogram with four right
angles. But any quadrilateral with four right angles is a
rectangle because any quadrilateral with four right angles
is a parallelogram.




Rhombus Corollary: A quadrilateral is a rhombus if and only if it
has four congruent sides.
Rectangle Corollary: A quadrilateral is a rectangle if and only if it
has four right angles.
Square Corollary: A quadrilateral is a square if and only if it is a
rhombus and a rectangle.
You can use these to prove that a quadrilateral is a rhombus,
rectangle or square without proving first that the quadrilateral is a
parallelogram.
EX. 3: USING PROPERTIES OF A RHOMBUS
THINK….INK…SHARE
P
Q
In the diagram at the right,
PQRS is a rhombus. What
is the value of y?
2y + 3

S
5y - 6
All four sides of a rhombus are ≅, so RS = PS.
5y – 6 = 2y + 3 Equate lengths of ≅ sides.
5y = 2y + 9 Add 6 to each side.
3y = 9
Subtract 2y from each side.
y=3
Divide each side by 3.
R
EXAMPLE 1
For any rhombus QRST, decide whether the statement is always or
sometimes true. Draw a sketch and explain your reasoning.
b.
Q
R
SOLUTION
b.
If rhombus QRST is a square, then
all four angles are congruent right
R if QRST is a
angles. So, Q
square. Because not all rhombuses
are also squares, the statement is
sometimes true.
EXAMPLE 2
SOLUTION
sides. One of the angles is not a right
angle, so the rhombus is not also a
square. By the Rhombus Corollary, the
GUIDED PRACTICE
for Examples 1 and 2
2. A quadrilateral has four congruent sides and four congruent
angles. Sketch the quadrilateral and classify it.
square
USING DIAGONALS OF SPECIAL PARALLELOGRAMS



The following theorems
rhombuses and
rectangles.
Theorem 8.11: A
parallelogram is a
rhombus if and only if its
diagonals are
perpendicular.
ABCD is a rhombus if and
only if AC BD.
EXAMPLE 3
List properties of special parallelograms
Sketch rectangle ABCD. List everything that you know
SOLUTION
By definition, you need to draw a figure
with the following properties:
• The figure is a parallelogram.
• The figure has four right angles.
Because ABCD is a parallelogram, it also has
these properties:
EXAMPLE 3
List properties of special parallelograms
• Opposite sides are parallel and congruent.
• Opposite angles are congruent. Consecutive angles
are supplementary.
• Diagonals bisect each other.
By Theorem 8.13, the diagonals of ABCD are congruent.
USING DIAGONALS OF SPECIAL PARALLELOGRAMS


Theorem 8.12: A
parallelogram is a
rhombus if and only if
each diagonal bisects a
pair of opposite angles.
ABCD is a rhombus if and
only if AC bisects DAB
and BCD and BD bisects
EXAMPLE 4
Solve a real-world problem
Carpentry
You are building a frame for a window.
The window will be installed in the
opening shown in the diagram.
a. The opening must be a rectangle.
Given the measurements in the
diagram, can you assume that it is?
Explain.
b. You measure the diagonals
of the opening. The diagonals
are 54.8 inches and 55.3 inches. What can you conclude about the shape of
the opening?
EXAMPLE 4
Solve a real-world problem
SOLUTION
a.
No, you cannot. The boards on opposite sides are the same length,
so they form a parallelogram. But you do not know whether the
angles are right angles.
b. By Theorem 8.13, the diagonals of a rectangle are
congruent.
formed by the boards are not
congruent, so the
boards do not form a rectangle.
GUIDED PRACTICE
for Example 4
4. Suppose you measure only the diagonals of a window opening. If
the diagonals have the same measure, can you conclude that the
opening is a rectangle? Explain.
yes, Theorem 8.13
USING DIAGONALS OF SPECIAL PARALLELOGRAMS
A


Theorem 8.13: A
parallelogram is a
rectangle if and only if its
diagonals are congruent.
ABCD is a rectangle if and
D
only if AC ≅ BD.
B
C
NOTE:

You can rewrite Theorem 8.11 as a
conditional statement and its converse.
Conditional statement: If the diagonals of a
parallelogram are perpendicular, then the
parallelogram is a rhombus.
Converse: If a parallelogram is a rhombus, then
its diagonals are perpendicular.
A
EX. 4: PROVING THEOREM 6.11
GIVEN: ABCD IS A RHOMBUS
PROVE: AC  BD
Statements:
1. ABCD is a rhombus
2. AB ≅ CB
3. AX ≅ CX
4. BX ≅ DX
5. ∆AXB ≅ ∆CXB
6. AXB ≅ CXB
7. AC  BD
B
X
D
Reasons:
1. Given
C
A
EX. 4: PROVING THEOREM 8.11
GIVEN: ABCD IS A RHOMBUS
PROVE: AC  BD
Statements:
1. ABCD is a rhombus
2. AB ≅ CB
3. AX ≅ CX
4. BX ≅ DX
5. ∆AXB ≅ ∆CXB
6. AXB ≅ CXB
7. AC  BD
B
X
D
Reasons:
1. Given
2. Given
C
A
EX. 4: PROVING THEOREM 8.11
GIVEN: ABCD IS A RHOMBUS
PROVE: AC  BD
Statements:
1. ABCD is a rhombus
2. AB ≅ CB
3. AX ≅ CX
4. BX ≅ DX
5. ∆AXB ≅ ∆CXB
6. AXB ≅ CXB
7. AC  BD
B
X
D
C
Reasons:
1. Given
2. Given
3. Def. of . Diagonals
bisect each other.
A
EX. 4: PROVING THEOREM 8.11
GIVEN: ABCD IS A RHOMBUS
PROVE: AC  BD
Statements:
1. ABCD is a rhombus
2. AB ≅ CB
3. AX ≅ CX
4. BX ≅ DX
5. ∆AXB ≅ ∆CXB
6. AXB ≅ CXB
7. AC  BD
B
X
D
C
Reasons:
1. Given
2. Given
3. Def. of . Diagonals
bisect each other.
4. Def. of . Diagonals
bisect each other.
A
EX. 4: PROVING THEOREM 8.11
GIVEN: ABCD IS A RHOMBUS
PROVE: AC  BD
Statements:
1. ABCD is a rhombus
2. AB ≅ CB
3. AX ≅ CX
4. BX ≅ DX
5. ∆AXB ≅ ∆CXB
6. AXB ≅ CXB
7. AC  BD
B
X
D
C
Reasons:
1. Given
2. Given
3. Def. of . Diagonals
bisect each other.
4. Def. of . Diagonals
bisect each other.
5. SSS congruence post.
A
B
EX. 4: PROVING THEOREM 8.11
GIVEN: ABCD IS A RHOMBUS
PROVE: AC  BD
Statements:
1. ABCD is a rhombus
2. AB ≅ CB
3. AX ≅ CX
4. BX ≅ DX
5. ∆AXB ≅ ∆CXB
6. AXB ≅ CXB
7. AC  BD
X
D
C
Reasons:
1. Given
2. Given
3. Def. of . Diagonals
bisect each other.
4. Def. of . Diagonals
bisect each other.
5. SSS congruence post.
6. CPCTC
A
EX. 4: PROVING THEOREM 8.11
GIVEN: ABCD IS A RHOMBUS
PROVE: AC  BD
Statements:
1.
ABCD is a rhombus
2.
AB ≅ CB
3.
AX ≅ CX
4.
BX ≅ DX
5.
∆AXB ≅ ∆CXB
6.
AXB ≅ CXB
7.
AC  BD
B
X
D
C
Reasons:
1.
Given
2.
Given
3.
Def. of . Diagonals bisect
each other.
4.
Def. of . Diagonals bisect
each other.
5.
SSS congruence post.
6.
CPCTC
7.
EX 6: CHECKING A RECTANGLE
THINK…INK…SHARE

a.
b.
CARPENTRY. You are
building a rectangular frame
for a theater set.
First, you nail four pieces of
wood together as shown at
the right. What is the shape
of the frame?
To make sure the frame is a
rectangle, you measure the
diagonals. One is 7 feet 4
inches. The other is 7 feet 2
inches. Is the frame a
rectangle? Explain.
4 feet
6 feet
6 feet
4 feet
EX 6: CHECKING A RECTANGLE
a.
First, you nail four
pieces of wood together
as shown at the right.
What is the shape of
the frame?
4 feet
6 feet
6 feet
Opposite sides are
congruent, so the frame
is a parallelogram.
4 feet
EX 6: CHECKING A RECTANGLE
To make sure the frame
is a rectangle, you
measure the diagonals.
One is 7 feet 4 inches.
The other is 7 feet 2
inches. Is the frame a
rectangle? Explain.
The parallelogram is NOT a
rectangle. If it were a
rectangle, the diagonals
would be congruent.
4 feet
b.
6 feet
6 feet
4 feet
WHAT WAS THE OBJECTIVE(S) FOR TODAY?
Students will analyze &amp; identify Rhombusses,
Rectangles &amp; Squares.
 Why? So you can solve carpentry problems, as
seen in EX 4.
 Mastery is 80% or better on 5-minute checks
and practice problems.

HOMEWORK
Page 537-539
 # 2-48 even &amp; 52 &amp; 53
