Compare to this post.

Are there any other mistakes that you would expect students to make on this problem?

Thanks to Kristen Fouss for the submission.

Compare to this post.

Are there any other mistakes that you would expect students to make on this problem?

Thanks to Kristen Fouss for the submission.

How do you teach this topic so that students don’t make this mistake? (Or do you just rail on their earlier teachers?)

Thanks to Kristen Fouss for the submission.

In the comments: can you reconstruct each step of the student’s argument? What was he thinking?

Thanks to Kristen Fouss for the submission.

[We have a growing library of trig identities, and I’ve got like 10 more trig identities to post over the next few weeks. Seems to be a tough spot for students.]

What’s up with the above? Say something smart in the comments about the implications of a mistake like this one. What’s another mistake that you would expect this student to make in the future? What’s a mistake that you wouldn’t expect the student to make?

Thanks to Jonathan Newman for the submission.

My sense is that this mistake isn’t as interesting as the rest, but it’s a pretty common one that I see in Trigonometry. The question is, what sort of activity would help this student out?