## All posts tagged **integral**

# Increasingly difficult integrals

*Posted by John Quintanilla on May 28, 2018*

https://meangreenmath.com/2018/05/28/increasingly-difficult-integrals/

# Borwein integrals

When teaching proofs, I always stress to my students that it’s not enough to do a few examples and then extrapolate, because it’s possible that the pattern might break down with a sufficiently large example. Here’s an example of this theme that I recently learned:

For further reading:

*Posted by John Quintanilla on February 5, 2018*

https://meangreenmath.com/2018/02/05/borwein-integrals/

# 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 , where is the region

There are two reasonable options for calculating this double integral.

- Option #1: Integrate with respect to first:

- Option #2: Integrate with respect to first:

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 Adventurebooks 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.

*Posted by John Quintanilla on May 12, 2017*

https://meangreenmath.com/2017/05/12/my-favorite-one-liners-part-101/

# My Favorite One-Liners: Part 88

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.

*Posted by John Quintanilla on April 29, 2017*

https://meangreenmath.com/2017/04/29/my-favorite-one-liners-part-88/

# My Favorite One-Liners: Part 87

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.

When teaching Calculus I, I use the following mantra throughout the semester. I heard this from my calculus instructor back in 1984, and I repeat it for my own students:

There are two themes of calculus: approximating curved things by straight things, and passing to limits.

For example, to find a derivative, we approximate a curved function by a straight tangent line and then pass to a limit. Later in the semester, to find a definite integral, we approximate the area under a curve by the sum of a bunch of straight rectangles and then pass to a limit.

For further reading, I’ll refer to this series of posts on what I typically do on the first day of my calculus class.

*Posted by John Quintanilla on April 28, 2017*

https://meangreenmath.com/2017/04/28/my-favorite-one-liners-part-87/

# My Favorite One-Liners: Part 74

After presenting the Fundamental Theorem of Calculus to my calculus students, I make a point of doing the following example in class:

Hopefully my students are able to produce the correct answer:

Then I tell my students that they’ve probably known the solution of this one since they were kids… and I show them the classic video “Unpack Your Adjectives” from Schoolhouse Rock. They’ll watch this video with no small amount of confusion (“How is this possibly connected to calculus?”)… until I reach the 1:15 mark of the video below, when I’ll pause and discuss this children’s cartoon. This never fails to get an enthusiastic response from my students.

If you have no idea what I’m talking about, be sure to watch the first 75 seconds of the video below. I think you’ll be amused.

*Posted by John Quintanilla on April 15, 2017*

https://meangreenmath.com/2017/04/15/my-favorite-one-liners-part-74/

# My Favorite One-Liners: Part 60

I’m a big believer using scaffolded lesson plans, starting from elementary ideas and gradually building up to complicated ideas. For example, when teaching calculus, I’ll use the following sequence of problems to introduce students to finding the volume of a solid of revolution using disks, washers, and shells:

- Find the volume of a cone with height and base radius .
- Find the volume of the solid generated by revolving the region bounded by , for , and the axis about the line .
- Find the volume of the solid generated by revolving the region bounded by , , and the axis about the line .
- Find the volume of the solid generated by revolving the region bounded by , , and the axis about the axis .
- Find the volume of the solid generated by revolving the region bounded by , , and the axis about the axis .
- Find the volume of the solid generated by revolving the region bounded by the parabola and the line about the line .
- Water is poured into a spherical tank of radius to a depth . How much water is in the tank?
- Find the volume of the solid generated by revolving the region bounded by , the axis, and about the axis.
- Find the volume of the solid generated by revolving the region bounded by , the axis, and about the axis.
- Repeat the previous problem using cylindrical shells.

In this sequence of problems, I slowly get my students accustomed to the ideas of horizontal and vertical slices, integrating with respect to either and , the creation of disks and washers and (eventually) cylindrical shells.

As the problems increase in difficulty, I enjoy using the following punch line:

To quote the great philosopher Emeril Lagasse, “Let’s kick it up a notch.”

*Posted by John Quintanilla on April 1, 2017*

https://meangreenmath.com/2017/04/01/my-favorite-one-liners-part-60/

# My Favorite One-Liners: Part 25

Consider the integral

The standard technique — other than multiplying it out — is using the substitution . With this substitution . Also, corresponds to , while corresponds to . Therefore,

.

My one-liner at this point is telling my students, “At this point, about 10,000 volts of electricity should be going down your spine.” I’ll use this line when a very unexpected result happens — like a “left” endpoint that’s greater than the “right” endpoint. Naturally, for this problem, the next step — though not logically necessary, it’s psychologically reassuring — is to absorb the negative sign by flipping the endpoints:

,

and then the calculation can continue.

*Posted by John Quintanilla on February 25, 2017*

https://meangreenmath.com/2017/02/25/my-favorite-one-liners-part-25/

# My Favorite One-Liners: Part 24

Here’s a problem that could appear in my class in probability or statistics:

Let be a probability density function for . Find , the cumulative distribution function of .

A student’s first reaction might be to set up the integral as

The problem with this set-up, of course, is that the letter has already been reserved as the right endpoint for this definite integral. Therefore, inside the integral, we should choose any other letter — just not — as the dummy variable.

Which sets up my one-liner: “In the words of the great philosopher Jean-Luc Picard: Plenty of letters left in the alphabet.”

We then write the integral as something like

and then get on with the business of finding .

*Posted by John Quintanilla on February 24, 2017*

https://meangreenmath.com/2017/02/24/my-favorite-one-liners-part-24/

# What I Learned from Reading “Gamma: Exploring Euler’s Constant” by Julian Havil: Part 5

Check out this lovely integral, dubbed the Sophomore’s Dream, found by Johann Bernoulli in 1697 (*Gamma*, page 44):

.

I’ll refer to either Wikipedia or Mathworld for the derivation.

When I researching for my series of posts on conditional convergence, especially examples related to the constant , the reference *Gamma: Exploring Euler’s Constant* by Julian Havil kept popping up. Finally, I decided to splurge for the book, expecting a decent popular account of this number. After all, I’m a professional mathematician, and I took a graduate level class in analytic number theory. In short, I don’t expect to learn a whole lot when reading a popular science book other than perhaps some new pedagogical insights.

Boy, was I wrong. As I turned every page, it seemed I hit a new factoid that I had not known before.

In this series, I’d like to compile some of my favorites — while giving the book a very high recommendation.

*Posted by John Quintanilla on October 5, 2016*

https://meangreenmath.com/2016/10/05/what-i-learned-from-reading-gamma-exploring-eulers-constant-by-julian-havil-part-5/