My Favorite One-Liners: Part 101

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.

I’ll use today’s one-liner when a choice has to be made between two different techniques of approximately equal difficulty. For example:

Calculate \displaystyle \iint_R e^{-x-2y}, where R is the region \{(x,y): 0 \le x \le y < \infty \}

There are two reasonable options for calculating this double integral.

  • Option #1: Integrate with respect to x first:

\int_0^\infty \int_0^y e^{-x-2y} dx dy

  • Option #2: Integrate with respect to y first:

\int_0^\infty \int_x^\infty e^{-x-2y} dy dx

Both techniques require about the same amount of effort before getting the final answer. So which technique should we choose? Well, as the instructor, I realize that it really doesn’t matter, so I’ll throw it open for a student vote by asking my class:

Anyone ever read the Choose Your Own Adventure books when you were kids?

After the class decides which technique to use, then we’ll set off on the adventure of computing the double integral.

This quip also works well when finding the volume of a solid of revolution. We teach our students two different techniques for finding such volumes: disks/washers and cylindrical shells. If it’s a toss-up as to which technique is best, I’ll let the class vote as to which technique to use before computing the volume.

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