Ripping the corners off a triangle to prove the sum of its interior angles is 180 degree is my favorite visual proof. Over the weekend, I posted a couple short videos for finding the sum of interior angles in a polygon, so here I wanted to share the videos and the worksheets from the videos.

Sum of interior angles - triangle and quadrilateral video |

Above is a short, 17-second video showing the sum of a triangle's interior angles is 180 degrees, and the sum of a quadrilateral's interior angles is 360 degrees.

Sum of interior angles of a pentagon video |

This second short shows the sum of the interior angles of a pentagon is 540 degrees. The angles needed a bit of trimming to fit together, but clearly showed the "circle and a half". How cool is it that we can show these proofs using cut paper?

If you'd like the triangle, quadrilateral and pentagon shapes worksheets from the videos, they are here in my Google Drive.

Geometry Word Wall reference |

A teacher emailed to ask if I had a math word wall reference for the sums of interior angles of polygons. I added the above reference to the Geometry Word Wall today.

Free math resource library |

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