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Illustrating the Sum of Angles in a Triangle

I created this interactive notebook page to help illustrate the sum of angles in a triangle.

Instead of just telling my students that the sum of the angles in a triangle is always 180 degrees (something I hope they remember from geometry!), I decided to have them cut out a triangle and illustrate it themselves.

We folded one angle down to the other side to create a fold that was parallel to the side we were folding towards.  Some students really struggled with this step!

Then, we marked where each angle ended up after being folded.

If students followed the directions properly, all of the angles should have formed a straight line!

Finally, I asked them what geometric concept this proved.  This was definitely not an original idea.  I found an example of folding a triangle to illustrate the fact that the sum of the angles in a triangle is 180 degrees in this blog post.