IM G Unit 1 Lesson 21 Cool Down

Elena wants to prove that the measures of the 3 angles in a triangle always sum to 180 degrees. She starts by using rigid motions to rotate triangle ABC 180 degrees around 2 midpoints.
Explain how Elena labeled her diagram and finish her proof.
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IM G Unit 1 Lesson 21 Cool Down from IM Geometry by Illustrative Mathematics, https://im.kendallhunt.com/HS/students/2/1/21/index.html. Licensed under the Creative Commons Attribution 4.0 license, https://creativecommons.org/licenses/by/4.0/.
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