My younger son’s “proof” of why the angles in a triangle sum to 180 degrees

Lovely moment with my younger son this morning.

We’ve just started the section about angles in Art of Problem Solving’s Prealgebra book. In solving some of the problems he sometimes used the idea that the angles in a triangle sum to 180 degrees. I’m sure that he’s just picked up this fact over the years from either our Family Math conversations or maybe just hearing my older son and I talk about geometry, but it isn’t something that we’ve actually studied, yet.

For today’s math project with him I asked him why he thought that fact was true, and he gave a wonderful reason. Not complete, but still really great. We then went on to talk through a more complete proof that hopefully helped him understand a bit more about angles and parallel lines.

Moments like this are why I love teaching my kids math.