# College Math Teaching

## August 28, 2017

### Integration by parts: why the choice of “v” from “dv” might matter…

We all know the integration by parts formula: $\int u dv = uv - \int v du$ though, of course, there is some choice in what $v$ is; any anti-derivative will do. Well, sort of.

I thought about this as I’ve been roped into teaching an actuarial mathematics class (and no, I have zero training in this area…grrr…)

So here is the set up: let $F_x(t) = P(0 \leq T_x \leq t)$ where $T_x$ is the random variable that denotes the number of years longer a person aged $x$ will live. Of course, $F_x$ is a probability distribution function with density function $f$ and if we assume that $F$ is smooth and $T_x$ has a finite expected value we can do the following: $E(T_x) = \int^{\infty}_0 t f_x(t) dt$ and, in principle this integral can be done by parts….but…if we use $u = t, dv = f_x(t), du = dt, v = F_x$ we have:

\

$t(F_x(t))|^{\infty}_0 -\int^{\infty}_0 F_x(t) dt$ which is a big problem on many levels. For one, $lim_{t \rightarrow \infty}F_x(t) = 1$ and so the new integral does not converge..and the first term doesn’t either.

But if, for $v = -(1-F_x(t))$ we note that $(1-F_x(t)) = S_x(t)$ is the survival function whose limit does go to zero, and there is usually the assumption that $tS_x(t) \rightarrow 0$ as $t \rightarrow \infty$

So we now have: $-(S_x(t) t)|^{\infty}_0 + \int^{\infty}_0 S_x(t) dt = \int^{\infty}_0 S_x(t) dt = E(T_x)$ which is one of the more important formulas.

## September 23, 2015

### Intelligence doesn’t show outwardly….

Filed under: academia, editorial — Tags: , — oldgote @ 12:02 pm

This semester has been the “semester from hell” in that I am teaching a class in actuarial mathematics and I have never seen the material before. So I am doing a “self-study” course on my own just ahead of the students.

I’ve done things like this before, but almost always it has been in classes where at least I understood both the notation and the point of the material fairly well.

The upside: I am learning something new.
But one consequence is that I have had little to share on this blog this semester.

I will make one comment though:

I am giving the first exam back in my “calculus II” (of 3) courses. This is the “off semester” which means that I’ll have students who placed out of calculus I and I’ll have those who have either flunked this course once (or several times) or I’ll have some who have been through our remedial calculus preparation program.

Hence, my grading curve looks like a “bathtub” curve.

But, time and time again, I am fascinated by the fact that all of the students, both the smart ones and the not-so-smart ones, “look alike” in that you can not distinguish them by appearance.

This is just the opposite from sports.

In a 5K race, if I see some tiny, slender but muscular person I know that I won’t see them after the start of the race. In the gym, if i see some guy who looks like he was carved out of marble, I know that I’ll be lifting about half of what he will.

But intelligence just doesn’t show in the same way.

## September 18, 2015

### Teaching a class that one is unqualified to teach…

Filed under: academia, editorial — Tags: , — oldgote @ 11:39 am

I haven’t posted much lately. I might post some this weekend, IF I ever get caught up. I have a couple of homework sets and one set of exams to grade.

What is going on: originally, I had a 3 preparation schedule: second semester calculus (the usual), first semester “business” calculus and numerical analysis. The latter is a time suck, but I’ve taught this course multiple times and have the details reasonably well worked out.

Note: my research specialty is topology though I’ve published an elementary analysis paper as well.

Anyway, it turns out that our part time instructor who teaches our “theory of interest” and “life contingencies” class got called away and we had no one to cover a class that had 18 students enrolled. A call for volunteers was put out and I said “if no one else….” BIG MISTAKE.

I am ok with it being an evening class.

But:

1. The amount of preparation time is incredible; basically I am teaching myself this material about 1 week (if that) ahead of walking into the class. I do ALL the homework to make sure I can do it correctly.
2. While the nuts and bolts are elementary on mathematical grounds, I have very little extra insight to offer. In the other classes, I can give a bit of perspective on “what is out there”. Not so in this class. I can teach “how to use the table”.
3. I am one who needs to know the stuff really, really well (at almost an unconscious level) to be comfortable in the class room. I don’t “fake it” well.

On the other hand, this is one of the things about having earned a Ph. D. and continuing to do research (though not this semester): I know how to learn and how to test my own knowledge on something. That is an ability that allows for me to “sub” in an area that I am not qualified to be in.

Of course, I still think that our university is obligated to hire a qualified person in this area if it wants to offer an actuarial program, though our increasingly corporate administration disagrees.