Reflection and Rotation
Symmetry
Mr. Belanger
Geometry – 9.4
Reflection-Symmetric Figures
A figure has symmetry if there is an isometry that maps
the figure onto itself. If that isometry is a reflection, then
the figure has reflection symmetry.
Activity 1:
Lines of symmetry can cut through shapes that
have reflection symmetry
Draw in lines of symmetry for each:
A
C
G
none
Segment Theorem:
Figures with reflection symmetry have their pre-images
and images equal distances from the reflection mirror.
5 in
6 in
10 in
5
6
10
Circle Symmetry:
How many lines of symmetry does a circle have??
Infinite!
Symmetric Figures Theorem:
Any symmetric figure is congruent to its image
Rotation Symmetry:
A figure has rotational symmetry if it’s congruent after a
rotation of 180 degrees or less (greater than zero).
Find the degrees of roation by
dividing 360 by number of
points.
360/3 = 120
Point Symmetry
A figure also has point symmetry if it can be rotated 180
degrees.
360/4 = 90 two rotation make 180
Examples:
Name the type(s) of symmetry each figure has.
Rotation of 120
reflection
Rotation
and
point
reflection