reflection and rotation symmetry mr. belanger geometry – 9.4

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