Thursday, April 14, 2011

Teachable Moments

or How a Conversation About Tall Mountains Turned Into a Lesson on Derivatives

I usually only share the good stuff and I think this qualifies.

I have an interesting group 3rd period. It's a geometry/exploratory math class and the kids in there are very curious. About everything. Today they were arguing about the world's tallest mountain and what considerations are used to determine height.

One student went to Wikipedia and pulled up this page and the highlighted graphic got us talking about relative maximum and minimum.

So I sketch a run-of-the-mill cubic function on the board and I ask what the maximum and minimum are.

"It goes on forever in both directions, Mr. Cox."

"Is it increasing or decreasing?"

"It goes up, then down, them up again."

"Ok, so what do these two points have in common?"
*pointing to relative max and min*


We talked about why the points were considered relative in terms of being a maximum or minimum and discussed how right before the relative max, the function is increasing. Right after the max it's decreasing.

They quickly connected their understanding of slope to the increasing or decreasing nature of the function. Then I pointed to the relative maximum and minimum and asked:

"So what's the slope here?"


"And, so what do they have in common?"

"Zero! The slope is zero at both points."

"How do you think we can figure the slope at different points on the graph?"

This time I sketched y = x2 and we talked about how we need two points to determine a slope.

"Would we be more accurate if the points are far apart or close together?"


"How close is close enough?"

It wasn't long before we had it.

4th quarter projects should be interesting.

- Posted using BlogPress from my iPad


Anne said...

seriously??? seriously!

Frank Noschese said...

While that's really cool, it's really inefficient. And what there the other students' doing while you were talking with those three? You should have given your students this: so they could compute derivatives using the exponenet rule like a bouss.

Jason Buell said...


I can't even process this.


It cracks me up that you'd that link. I dub that "kahnrolling"

Sue VanHattum said...

Way cool. You know you love math when you find yourself getting the most unlikely audience to ask questions about calculus.

Roy Wright said...

What a wonderful coincidence. I have been intensely wondering lately why the simple and elegant idea of differentiation isn't introduced in algebra, instead of waiting for the poor students to climb a mountain of tedious and mostly unnecessary "prerequisites" on the way to calculus.

David Cox said...

@ Sue
These kids are very curious. It just kind of happens.

@Prof. Wright
I agree. I don't think I truly understood a derivative as a slope until I finished my calculus series.