geometry rotations. 10/19/2015 goals identify rotations in the plane. apply rotation formulas to...
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Geometry
Rotations
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Goals
Identify rotations in the plane. Apply rotation formulas to figures
on the coordinate plane.
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Rotation
A transformation in which a figure is turned about a fixed point, called the center of rotation.
Center of Rotation
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Rotation
Rays drawn from the center of rotation to a point and its image form an angle called the angle of rotation.
Center of Rotation
90
G
G’
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A Rotation is an Isometry
Segment lengths are preserved. Angle measures are preserved. Parallel lines remain parallel. Orientation is unchanged.
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Rotations on the Coordinate Plane
Know the formulas for:
•90 rotations
•180 rotations
•clockwise & counter-clockwise
Unless told otherwise, the center of rotation is the origin (0, 0).
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90 clockwise rotation
Formula
(x, y) (y, x)A(-2, 4)
A’(4, 2)
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Rotate (-3, -2) 90 clockwise
Formula
(x, y) (y, x)
(-3, -2)
A’(-2, 3)
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90 counter-clockwise rotation
Formula
(x, y) (y, x)
A(4, -2)
A’(2, 4)
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Rotate (-5, 3) 90 counter-clockwise
Formula
(x, y) (y, x)
(-3, -5)
(-5, 3)
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180 rotation
Formula
(x, y) (x, y)
A(-4, -2)
A’(4, 2)
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Rotate (3, -4) 180
Formula
(x, y) (x, y)
(3, -4)
(-3, 4)
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Rotation Example
Draw a coordinate grid and graph:
A(-3, 0)
B(-2, 4)
C(1, -1)
Draw ABC
A(-3, 0)
B(-2, 4)
C(1, -1)
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Rotation Example
Rotate ABC 90 clockwise.
Formula
(x, y) (y, x)A(-3, 0)
B(-2, 4)
C(1, -1)
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Rotate ABC 90 clockwise.
(x, y) (y, x)
A(-3, 0) A’(0, 3)
B(-2, 4) B’(4, 2)
C(1, -1) C’(-1, -1)A(-3, 0)
B(-2, 4)
C(1, -1)
A’B’
C’
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Rotate ABC 90 clockwise.
Check by rotating ABC 90.
A(-3, 0)
B(-2, 4)
C(1, -1)
A’B’
C’
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Rotation Formulas 90 CW (x, y) (y, x) 90 CCW (x, y) (y, x) 180 (x, y) (x, y)
Rotating through an angle other than 90 or 180 requires much more complicated math.
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Compound Reflections
If lines k and m intersect at point P, then a reflection in k followed by a reflection in m is the same as a rotation about point P.
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Compound Reflections If lines k and m intersect at point P, then a reflection in
k followed by a reflection in m is the same as a rotation about point P.
P
mk
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Compound Reflections Furthermore, the amount of the rotation is
twice the measure of the angle between lines k and m.
P
mk
45
90
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Compound Reflections The amount of the rotation is twice the
measure of the angle between lines k and m.
P
mk
x
2x
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Rotational Symmetry
A figure can be mapped onto itself by a rotation of 180 or less.
4590
The square has rotational symmetry of 90.
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Does this figure have rotational symmetry?
The hexagon has rotational symmetry of 60.
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Does this figure have rotational symmetry?
Yes, of 180.
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Does this figure have rotational symmetry?
No, it required a full 360 to map onto itself.
90
180
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Rotating segments
A
B
C
D
E
F
G
H
O
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Rotating AC 90 CW about the origin maps it to _______.
A
B
C
D
E
F
G
H
CE
O
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Rotating HG 90 CCW about the origin maps it to _______.
A
B
C
D
E
F
G
H
FE
O
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Rotating AH 180 about the origin maps it to _______.
A
B
C
D
E
F
G
H
ED
O
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Rotating GF 90 CCW about point G maps it to _______.
A
B
C
D
E
F
G
H
GH
O
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Rotating ACEG 180 about the origin maps it to _______.
A
B
C
D
E
F
G
H
EGAC
A E
C
G
O
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Rotating FED 270 CCW about point D maps it to _______.
A
B
C
D
E
F
G
H
BOD
O
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Summary
A rotation is a transformation where the preimage is rotated about the center of rotation.
Rotations are Isometries. A figure has rotational symmetry if
it maps onto itself at an angle of rotation of 180 or less.
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Homework