The Line of Reflection

10 Mar

I’ve blogged about the Illustrative Mathematics task Reflected Triangles before. I really like that it asks students to determine the line of reflection given the pre-image and image instead of determining the image given the pre-image and line of reflection.

△ABC has been reflected across a line into the blue triangle. Construct the line across which the triangle was reflected. Justify your conclusion.

1 03-10-2015 Image002

How would you construct the line of reflection?

Work on your mathematical flexibility to come up with more than one way to construct the line of reflection.

2 MathFlex_Alg

I used Class Capture to monitor student work and selected some students to share their approach with the whole class.

2 Screen Shot 2014-08-28 at 9.30.19 AM

Many students approached the task like Paris:

3 Screen Shot 2014-08-28 at 9.42.23 AM

What can we learn about the relationship between the pre-image, image, and line of reflection from the additional line in Max’s diagram?

4 Screen Shot 2014-08-28 at 9.42.53 AM

I’ve always used this task towards the end of our unit on Rigid Motions, but I am thinking about using it earlier in the unit next year. Maybe the task itself could be an #AskDontTell approach for students learning what we want them to learn about the relationship between the pre-image, image, and line of reflection. We will see next year, as the journey continues …

Leave a comment

Posted by on March 10, 2015 in Geometry, Rigid Motions


Tags: , , , , ,

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: