Why did this student think that this verified the identity?

(Thanks Michelle!)

What’s the fastest way to help this student?

Do you have a good way to help students understand the rigor needed in these sort of proofs?

Thanks to Jonathan Newman for the submission.

This is a fun mistake, via Jonathan Newman. Find it, and then explain how you’d help the kid who made it.

What’s the fastest way of helping this student?

Thanks to Jonathan Newman for the submission. You should click on his name to go to his blog.

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.]