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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.

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Many students approached the task like Paris:

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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 …

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

 

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