In this series, I’m compiling some of the quips and one-liners that I’ll use with my students to hopefully make my lessons more memorable for them.

In the first few weeks of my calculus class, after introducing the definition of a derivative,

,

I’ll use the following steps to guide my students to find the derivatives of polynomials.

- If , a constant, then .
- If and are both differentiable, then .
- If is differentiable and is a constant, then .
- If , where is a nonnegative integer, then .
- If is a polynomial, then .

After doing a few examples to help these concepts sink in, I’ll show the following two examples with about 3-4 minutes left in class.

Example 1. Let . Notice I’ve changed the variable from to , but that’s OK. Does this remind you of anything? (Students answer: the area of a circle.)What’s the derivative? Remember, is just a constant. So .

Does this remind you of anything? (

Students answer: Whoa… the circumference of a circle.)

Generally, students start waking up even though it’s near the end of class. I continue:

Example 2. Now let’s try . Does this remind you of anything? (Students answer: the volume of a sphere.)What’s the derivative? Again, is just a constant. So .

Does this remind you of anything? (

Students answer: Whoa… the surface area of a sphere.)

By now, I’ve really got my students’ attention with this unexpected connection between these formulas from high school geometry. If I’ve timed things right, I’ll say the following with about 30-60 seconds left in class:

Hmmm. That’s interesting. The derivative of the area of a circle is the circumference of the circle, and the derivative of the area of a sphere is the surface area of the sphere. I wonder why this works. Any ideas? (

Students: stunned silence.)This is what’s known as a cliff-hanger, and I’ll give you the answer at the start of class tomorrow. (

Students groan, as they really want to know the answer immediately.) Class is dismissed.

If you’d like to see the answer, see my previous post on this topic.