Download Properties of Rotations, Reflections, and Translations

LE
SS
O
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18
Properties of Rotations, Reflections,
and Translations
UNDERSTAND A rigid motion changes the position of a figure without changing
its shape or size. A sequence of rigid motions can transform a figure
into a congruent figure. Each point on the original figure matches
a corresponding point on the transformed figure, called the image. If the vertices of the original figure are labeled with letters, such as
ABCD, then the vertices of the image will be labeled with a prime
symbol, (9), as in A9B9C9D9.
A rotation is a turn of a figure about a point. Triangle J9K9L9 is a 90°-clockwise rotation of triangle JKL
about point J.
K
J
L
K
L
A reflection is a flip of a figure over a point or line. Quadrilateral M9N9O9P9 is a reflection of quadrilateral MNOP
over the vertical line.
N
P P
O
A translation is a slide of a figure to a new location. Pentagon A9B9C9D9E9 is a translation of pentagon ABCDE
4 units down and 5 units to the left.
N
M M
O
A
E
B
D
C
E
B
D
C
You can demonstrate the congruence of two figures by using a rigid motion or a sequence
of rigid motions to make the figures coincide.
104 Domain 4: Geometry
Duplicating any part of this book is prohibited by law.
A
Connect
Describe the rigid motion that will make the two intersecting lines coincide.
P
1
Trace the vertical line on tracing paper. 2
Rotate it around point P so that it
completely covers the slanted line.
45°
P
P
A clockwise rotation of 45° is needed. You can use a protractor to verify the
degree of rotation.
▸ A 45°-clockwise rotation around
point P can be used to make the
two lines coincide.
Describe a rigid motion or sequence of rigid motions
that could make the two line segments coincide.
Duplicating any part of this book is prohibited by law.
Trace the line segments. Fold the
tracing paper so that one segment
folds onto the other.
The line formed when the paper is
folded is a line of reflection.
TrY
Describe another way to use one or
more rigid motions that could make
the two line segments coincide. Trace the figure and use the tracing
to see if your way works.
▸ One way to make the line segments
coincide is to reflect either one onto
the other over a line of reflection.
Lesson 18: Properties of Rotations, Reflections, and Translations 105
EXAMPLE A Describe a sequence of rigid motions that could be used
to make the 60° angles coincide.
1
Which rigid motions could be used?
Are the angles turned differently? Yes
Are the angles mirror images of one
another? No
Are the angles in different locations? Yes
Try translating the top angle down so
the vertices coincide. Then rotate the
image until the angles coincide.
2
Use tracing paper. Trace the top angle
and move the tracing.
Translate the angle tracing down
5 units so that the vertices of the angles
coincide.
3
Rotate the tracing clockwise about the
common vertex of the angles.
▸ A translation of the top angle 5 units
down, followed by a 90°-clockwise
rotation around the vertex, makes the
angles coincide.
106 Domain 4: Geometry
DIS
CU S S
If you reverse the order of the two rigid
motions (first rotate, then translate),
would the angles coincide? Explain
why or why not.
Duplicating any part of this book is prohibited by law.
90°
EXAMPLE B Describe how a reflection and a translation could be used
A
to make parallel lines AB and CD coincide with parallel
lines A0B0 and C 0D 0.
1
C
D
A
‹___›
‹___›
D
B
Reflect AB​ ​     and CD​
​     over a line to create
a mirror image of the two lines.
A
B
C
D
C
D A
C
D
B
B
A
2
Lines AB and CD can be reflected over
a horizontal line to form lines A9B9 and
C9D9. Point A corresponds to point A9,
point B corresponds to point B9, point
C corresponds to point C9, and point D
corresponds to point D9.
Look at the diagram below.
A
C
C
A
TrY
Duplicating any part of this book is prohibited by law.
C
B
Describe how a reflection and a
translation could be used to make
parallel lines AB and CD coincide with
parallel lines A0B0 and C 0D 0. Use Math
Tool: Grid Paper to sketch the steps.
A
C
B
D
C
D A
D
B
B
Visualize sliding line A9B9 and line C9D9
so that each point moves 5 units to the
right and 3 units up to make parallel
lines A9B9 and C9D9 coincide with parallel
lines A0B0 and C 0D 0.
▸ A reflection across a horizontal
line below line CD, followed by a
translation to the right and up, makes
parallel lines AB and CD coincide with
parallel lines A0B0 and C 0D 0.
C
B
D
D
A
B
Lesson 18: Properties of Rotations, Reflections, and Translations 107
Practice
Identify the type of rigid motion needed to make the two figures coincide in one step.
1.
2.
3.
___
Identify the line segment that could coincide with AB​
​   if the rigid motions described
are performed.
A
C
B
G
E
D
F
___
4. AB​
​   is rotated 180°
around point B and
then translated down.
H
K
J
___
5. AB​
​   is rotated 90°
clockwise around
point A and then
reflected across a
vertical line.
___
6. AB​
​   is rotated 135°
clockwise around
point A and then
translated down.
Write true or false for each statement. If false, rewrite the statement so it is true.
7.
Rigid motions do not change the shape of a figure. 8.
Rigid motions may change the size of a figure. 108 Domain 4: Geometry
Duplicating any part of this book is prohibited by law.
REMEMBER You can use
tracing paper and a protractor
to help visualize the rigid
motions described.
Describe in detail how one or more rigid motions could be used to make the leftmost figure
coincide with the rightmost figure. Use the fewest rigid motions possible.
9.
A
B
B
10.
F
F
A
D
11.
E
E
D
12.
L
K
Y
L
X
Z
Y
K
Z
X
Solve.
14. WRITE MATH Reflect the pair of parallel
lines over the dashed line. Then explain
how a rotation and a reflection can be
used to move those reflected images
so that they coincide with the original
parallel lines.
Duplicating any part of this book is prohibited by law.
13. DESCRIBE Use words and/or drawings
to describe two different sequences
of rigid motions that could be used to
make the 50° angles shown coincide.
Lesson 18: Properties of Rotations, Reflections, and Translations 109