Prove that sqrt(2/3) is irrational.

A student emailed me the folloing (I paraphrase and am prob not as elegant or as long as he was)

Dr. Gasarch

I received only 2 points out of 20 on Problem 3 of the final- the one that asked us to prove that sqrt(2/3) is irrational. This was graded rather harshly, and the reason is endemic to the entire course. It was unclear what we could and could not assume in this problem. That has been unclear all semester.

I responded (I leave out the name for privacy)

Mr. Student

Come by my office and we will look over your final. At the same time, bring by your HWs and Midterms to look at. The deadline for regrading those is up; however, either you are correct and I will learn how to run a better course by looking over your HWs and midterms, or you incorrect and you will be enlightened by us looking over them.

We agreed to meet. The Student came all ready to argue not just points on the final but also about the course in general. I looked forward to this since either I would learn about how to run the course better OR I would enlighten a student!

STUDENT: I used the obvious fact that the ratio of two irrationals is irrational. How was I supposed to know I had to prove something so obvious!!!!!!!! And only 2 points!!!!!!! Really!!!!!!

BILL: What is the ratio of \sqrt{8} to \sqrt{2}.

STUDENT: sqrt(8)/sqrt(2) = sqrt(8/2) = sqrt(4) = 2. Why is that relevant?

BILL: You just showed that the ratio of two irrationals could be rational.

STUDENT: Oh.

BILL: Lets look over your HW and midterm and see there were times when it was not clear what you could assume OR if not then I can clear up some misconception you had.

STUDENT: Uh, Uh. Nevermind.

BILL: You are here with your HWs and midterms, lets take a look.

He really didn't want to. I think he really just wanted more points on the final But since he phrased it as a philosphical debate about how the course was run, I took him at his word. Everything he showed me was either clearly wrong or clearly unclear. None of it fell into the category of `not clear what you can assume'.

This disappointed me. I suspect someone could make a case that my course sometimes does not make it clear what you can assume. Other students with a similar story as above claim my course is to pedantic. But the examples they show me of this are usually just wrong, not marked wrong for being to pedantic.

There is a more general problem here. The students who complain about a course

*may well have a valid point to make!*But its usually students who are not very good making the complaint, and hence they are not the ones who could make a good argument. One thing I have done when a student has a complaint about how I run the course is then ask my TAs about it. This has been helpful sometimes but they are on the other end -- the course is easy for them so its hard for them to see the problems.

Having said all of this I will own up to one flaw I've had which the students complained about incoherently (see here) and my TA pointed out the fair objection. I had taught ONE type of Pigeon hold argument and tested them on a very closely related but different type -- as a way of testing if they understood pigeon hole and were not just memorizing. It was a fair question BUT my TA said

(correctly) I was more interested in getting a good test question then, you know, TEACHING them the Pigeon hole principle -- so I should in the future (and I have) teach them LOTS of versions, make sure they understand them, and then if on the exam I give them a variant its more fair. But more importantly he pointed out that I (and this is correct, or was) have a QUESTION I really want to put on the midterm and then teach the course so that it makes sense to do so. The question is fair, but this is NOT the point (which is why the students objections were incoherent- the question was fair). I am setting (some of them) up to fail. I have CHANGED my Teaching style and exam style since them.

But my point is that the students could not possibly have raised that objection-- partly because the students complaining are not very good, but also because they do not see what goes on behind the scenes.

UPSHOT- if your students have an incoherent complaint there may be something to it and you should ask your TAs.

## No comments:

## Post a Comment