What “critical thinking” might mean in math

Once again, a possible big lesson from the latest calculus homework set. One exercise gave the students this limit:


They were asked to figure this limit three ways: By graphing the function in a small window around x = 0 and estimating the limit; by making a table of values for the function near x = 0; and by doing some fancy algebra tricks to compute the limit exactly.

Most students successfully estimated the limit to be about 0.2887 using the graph and again using the table. (Although a lot of students figured that they didn’t need to include the graph they used, I guess because I should take their word for it?) To repeat: in parts (a) and (b), the students prove fairly conclusively that the limit does in fact exist and is equal to something like 0.2887.

Then they try it the algebra way, and all hell breaks loose. People squared the top and bottom of the fraction, thinking that this is a rule. (Question: Is 2/3 the same as 4/9?) Then they carry out the squaring in the numerator to get 3 + x – 3. Or perhaps they ignore the stern warning I gave them about just taking the limit of top and bottom of a fraction without first checking to see if the denominator vanishes. One way or another, they get that the limit fails to exist. In complete contradiction to their first two calculations.

Critical thinking in this situation would lead a student to have the following thought: Wait a minute. I got the limit to fail to exist. But that can’t possibly be right, because my first two calculations are telling me that there is a limit, and it’s equal to about 0.2887. Clearly something isn’t right with my algebra. I had better go back and fix it.

Uncritical thinking in this situation would give us this: Done with that one. On to the next one.

[Update: Or, suppose you found the instantaneous velocity of an arrow shot upward on the moon at a certain time to be, say, 57,942 meters per second. Yes, that’s a comma and not a decimal point. That speed is equal to 129,613 miles per hour. The speed of sound at sea level is 761 miles per hour. In other words, this arrow is moving a little over 170 times the speed of sound. Something in you should react to this, saying, “WTF have I done?” And then you go back and fix it. ]

I’m thinking more and more that “critical thinking” is nothing more than having an intuition about when something can’t possibly be right, having the good sense to pay attention to that intuition, and then having the combination of skill and desire to get things right which compels the student to go back and get his or her hands dirty. In fact I think only a part of “critical thinking” involves thinking — a good deal of it appears to be a combination of intuition and intellectual courage.

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Filed under Calculus, Critical thinking, Education, Higher ed, Math, Student culture, Teaching

5 responses to “What “critical thinking” might mean in math

  1. Eric

    I think that your definition of “critical thinking” is pretty accurate. The trick is how to teach students to have good intuitions, because many of them have a whole mess of bad ones.

    Oh, and by the way, 10 years later and I still remember enough calculus to apply L’Hospital’s rule. I must have been awake that morning. 🙂

  2. JimMc

    In addition to critical thinking, which you’re absolutely right about, some minimum amount of caring or interest on the student’s part might be missing too. You can be a critical thinker but rarely apply it to things you don’t care about (I’m plenty guilty of that). If a student just doesn’t give a hoot about math, then they’re just sunk.

  3. Jami

    I have to agree with JimMc. It probably isnt the critical thinking that most students are missing. I can remember countless times where I came up with an answer that I knew didnt make any sense, but I gave up because I couldnt figure out how to fix it.
    Now in response to that I know that you’ll say this was a homework set so the students had plenty of time and resources to fix this issue. But I think that might be more of the problem than the critical thinking issue. My guess is 1/2 of them just dont really care, or they are too shy (or whatever quality it is) to get help.

  4. I coined the phrase “intellectual courage” at the end of the post to describe what JimMC and Jami are saying. It’s the quality that makes a person take action on something intellectual or academic they did which they know isn’t right, getting help when it’s obviously needed despite one’s level of shyness or scaredness or whathaveyou — and if not actually *caring* about the work being done, at least realizing that getting the work done right is more important than one’s feelings about it.

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