When do Multiple Rotations Exist?

Published by MrHonner on

I recently profiled an erroneous high-stakes math exam question that had two correct answers.

January 2015 GEO 27

Here, it is possible to map AB onto A’B’ using either a glide reflection or a rotation.

It’s interesting to note that there are actually two distinct rotations that map AB onto A’B’, as demonstrated below.

Regents Question -- Two Rotations

This raises an interesting question:  given two congruent objects, under what circumstances will two distinct rotations exist that map one onto the other?

In a comment on the original post, Joshua Greene offered another interesting follow-up question:

Under what circumstance, if any, are two line segments of equal length not images of each other under rotation? In which of those cases, if any, are the two line segments images of each other under glide reflection?

With enough work, even erroneous exam questions are redeemable!


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