Geometric Measurement and Dimension Pythagorean Theorem Right Triangles Similarity, Right Triangles and Trigonometry

All Ramps Are 45 Degrees + Pythagorean Theorem

Two interesting mistakes here. The first has to do with the Pythagorean Theorem, the other (more interesting) has to do with the angle of inclination.



I wonder what she’s looking at that the angle always stays the same. My guess, based on her first triangle, is that she thinks that the diagonal of a rectangle always bisects the right angle.

This might make for a nice bit of feedback for her. I could ask, “Is it possible to draw a rectangle whose diagonals don’t always make 45 degree angles? The answer matters for what you wrote here.” Or maybe the feedback I supply here should be a counterexample — a very long rectangle whose diagonals clearly don’t make 45 degrees? What’s my goal in this feedback, anyway?

I suppose my only goal is to have her know that the diagonals don’t bisect the angles, and to believe this in a way that she’ll remember and be able to reproduce on a new problem. So I want to equip her with the means to prove it to herself.

Given all this, I think I should probably be more direct in my feedback about the fact of non-bisection. I should leave the proof up to her, though. “Try to draw a rectangle whose diagonals don’t make 45 degree angles.”

One last worry. What if I’m wrong about my diagnosis of her thinking? What if she is seeing 45 degrees in these ramps in some other way? Maybe the best thing is to check in with her verbally before giving her any written feedback, to confirm that my theory is correct?

Update (4/23/15): Here’s the feedback and her post-feedback work. In conversation, I was able to confirm that my “every rectangle’s diagonals bisect a right angle” theory was right.


Pythagorean Theorem Right Triangles Similarity, Right Triangles and Trigonometry Trigonometry

30/60/90 Mistakes

Right Triangle Quiz Responses


This is fairly representative of the class’ work. What would your next step be with this class?

Area Geometry High School: Geometry Right Triangles

Using a bad base



I keep on seeing this in my Geometry classes this year. Tasked with finding the area of a right triangle, kids move toward the hypotenuse even if two of the other sides are given. Then they end up stuck looking for a height that they can’t find.

I’m pretty convinced — based on talking to kids and looking at their work — that this is all about how they see right triangles. These kids must be seeing hypotenuses as bases, and it must feel weird for them to treat the legs as bases. Or maybe instead it’s about the height? Maybe it feels strange to them to use a leg as a height?

Geometry Right Triangles

Special Right Triangles

4 1 2 3

Let’s take for granted that these students don’t have conceptual understanding of the Pythagorean Theorem, because if they did, then they wouldn’t make these mistakes. (I actually think that we need to be more careful with the ways that we toss around phrases like “conceptual understanding” but whatever.)

What do these mistakes reveal about how these kids think about right triangles and the Pythagorean Theorem in the absence of conceptual understanding? Why does this ever make sense to the student?

Thanks to Michael Fenton for the submission!

Geometry Geometry Pythagorean Theorem Right Triangles

Sqrt(1) and Right Triangles


This mistake seems ripe for theorizing. Would a kid make this mistake if the hypotenuse was 4 instead of 10?

Geometric Measurement and Dimension Pythagorean Theorem Right Triangles Similar Figures Similarity, Right Triangles and Trigonometry

A whole bunch of questions about right triangles

geom25 question


geom 25


geom 25 2 (800x648)


What other mistakes would you expect to see from this problem? How do you teach so as to help students avoid these pitfalls?

Thanks to Tina Cardone for the submission.