Geometry Session 7: Symmetry

Session 7, Part A:
Line Symmetry

In This Part: Finding Lines of Symmetry | The Perpendicular Bisector

If point A' is the mirror image of point A in a figure with a line of symmetry, then the line of symmetry is the perpendicular bisector of the segment AA'.

You can use that fact to reflect figures over lines without using a device like a Mira. Suppose you want to reflect the triangle below over the line shown. To reflect point A, draw a segment from A perpendicular to the line. Continue the segment past the line until you've doubled its length. You now have point A', the mirror image of A.

Repeat the process for the other two endpoints, and connect them to form the triangle.

This interactive illustration requires the Flash plug-in, which you can download for free from Macromedia's Web site.

Problem A3

For each figure, reflect the figure over the line shown using perpendicular bisectors. Check your work with a Mira.

Printable PDF of figures

Video Segment
In this video segment, the participants explain different ways in which they came up with perpendicular bisectors to reflect the figures over the line of symmetry.

Did you use the same method as shown in the video segment? Did you come up with a different way of solving the problems?

If you are using a VCR, you can find this segment on the session video approximately 11 minutes and 9 seconds after the Annenberg Media logo.

Problem A3, the preceding text, and the Video Segment problem adapted from IMPACT Mathematics, Course 3, developed by Educational Development Center, Inc. pp. 295-296. © 2000 Glencoe/McGraw-Hill. Used with permission. www.glencoe.com/sec/math

Next > Part B: Rotation Symmetry

Session 7: Index | Notes | Solutions | Video