reflection and rotation symmetry mr. belanger geometry – 9.4
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Reflection and Rotation Symmetry
Mr. Belanger
Geometry – 9.4
Reflection-Symmetric FiguresA 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:
ACG 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 SymmetryA 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