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4th Grade Math – Classifying Triangles

Clip 12/18: Lesson Part 2D

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Hillary Lewis shifts the classroom discussion away from right isosceles triangles to acute isosceles triangles. She asks the students, “I'm wondering if you were able to make any acute isosceles triangles. Were you able to make any? Right now, let's see if you can make any more acute triangles and check them with your partner to make sure that you have it right.”

Hillary closes the lesson by asking students if they can make a triangle with three obtuse angles. The students discuss and quickly decide that it would be impossible.

One student comments, “If you go, yeah, like with two, then you need another side to connect them and that would make that a quadrilateral. Even if you made a right angle, they're still not going to connect. Even if it's barely slanted.”

Another student observes that “So if all them had 90 degrees, then if you add them all up it would be 270 degrees, which is over 180 degrees, but it has to be … 180 degrees if it's a triangle. So, if it's all obtuse, it'd have to be, all of the angles more than 90 degrees, so then you would be way over 270, which is over 180.”

Teacher Commentary

Hillary Lewis

The focus of this part of the lesson was having them create/construct different types of triangles from other geometric figures. That was what they were doing at the end with the straight edges. But there were other things that came up that I wasn't expecting.

Some were confused by the acute angles, and I think there were some kids still struggling with the idea of, What is an angle? Because that wasn't something I'd prepared for, and it wasn't my classroom, and I had all these observers there, I really didn't know how to deal with it in that moment, and so I wasn't able to dig into that in that moment.