Please Teach the Vertical Line Test

I was re-writing an introductory lesson on functions this morning and was reminded of something from years ago.

An influential teacher was telling their followers not to teach the vertical line test because it was confusing. I strongly disagreed. The vertical line test is a great way to meaningfully connect several fundamental ideas: the definition of a function, the definition of a graph, domain and range. The influencer was unmoved, but conceded slightly, saying that the topic should be handled with caution. I responded, “Yes, any time a teacher teaches something they don’t fully understand themselves, they should be cautious!”

It’s sad to know that there are teachers out there not teaching the vertical line test because someone told them it’s too confusing.

Originally posted on Mastodon.