
A cool student question
I was teaching the very beginnings of the idea of how to find the slope of a tangent line at (1,1) for the function f(x) = x^2 last Thursday.
We did it by 'creeping' up to x = 1 from both ends, like.. Idk it's hard to describe without my mouth or the ability to write on a whiteboard.
"We could hold the point (1,1) fixed and 'drag' the point (3, 9) to the left towards (1,1) until we got really close, like (1.01, 1.0201), and find the slope between those two points. That would be a really good approximation."
*makes table with x = 1.2, 1.1, 1.01*
"We could do the same from the under-side as well. Hold (1,1) fixed and slide (0,0) up and right really close to (1, 1), like (0.99, 0.9801) and find that slope."
Q: What does it appear the slope should be?
Class A: "It looks like the slope should be 2!
But then a student of mine asked a fun question: What if we took a point on either slide and slid both of those towards (1,1)? Would that work?
I had to pause for a bit and think about it. My answer was that having two moving parts might be a bit more cumbersome than needed, but yes it should work.
A few days later (today), the thought popped back into my head, so I did it. It works and it's simple and it's cool.