51 votes
Accepted

Should students be told they're wrong

All four of your options lead with "They are told..." Consider asking the student questions instead. At the very least, this shows interest, and they may end up catching their own mistakes as they try ...
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  • 606
43 votes
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How does a teacher come up with plausible wrong answers for multiple choice tests?

I freelance as an item writer, someone who writes questions for standardized tests. When making up alternate choices, I always have to justify my reasons for the "wrong answers" or distractors. Here ...
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  • 6,795
30 votes

Misuse of parentheses for multiplication

It is actually wrong to say that parenthesis means multiplication. In $(2)(5)$ it is the lack of an operator between the parenthesis that implies multiplication, NOT the parenthesis. The parenthesis ...
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  • 401
29 votes

How to handle the situation where a student insists I am wrong during the class?

I agree with @kathleen, that being a young (and female) instructor can lead some students to be less respectful. In this case, we can use that to our advantage... Long ago, when I was less secure as ...
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  • 17.3k
29 votes
Accepted

Near-universal student mistake on $\lim_{x\rightarrow\infty}e^{x+1}/e^x$

While all of your students at this point will have done (extensive) units on manipulating and simplifying expressions with exponents, this is the limits unit. When doing limits questions, most ...
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  • 4,850
27 votes
Accepted

Grating mathematical phrases---How to correct?

Personally, I don't think we attend to this sufficiently in lower-level mathematics (where it's actually needed most). Students need that vocabulary to interface with books, future teachers, tutors, ...
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27 votes

How should a student's inefficient calculation be pointed out?

Foremost: It depends on what the lead-in lesson/topic/direction was. If this was the essential point being exercised, then I would interrupt ASAP and refocus them on the lesson/direction that just ...
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27 votes

How to explain what's wrong with this application of the chain rule?

The root of the difficulty is that $x$ appears free in $f(z)$, but we are trying to "capture" it with $g(x)$, which is illegal. When we substitute $g(x)$ into $f(g(x))$, we have a variable clash: $$ f(...
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  • 556
26 votes
Accepted

Quote to show students don't have to fear making mistakes

Johann Wolfgang von Goethe: "By seeking and blundering we learn." Original German, 1825. Albert Einstein: "Anyone who has never made a mistake has never tried anything new." (However, attribution to ...
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22 votes

teach that $\frac10$ not defined properly

What is $\frac 1 a$? It is the unique (real) number such that $a\cdot \frac 1 a=1$. Does there exist a real number that multiplied by $0$ gives $1$? No. Why is this? Because if $0\cdot b=0$ which ever ...
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22 votes

How does a teacher come up with plausible wrong answers for multiple choice tests?

If a teacher has taught the course before, and has asked questions that are free-response (not multiple-choice), then the teacher can look at the incorrect answers previously given by the students. ...
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  • 10.2k
22 votes
Accepted

How do you coach students who often make small errors?

Ask the student to "talk through" their calculations Having a student verbalize their calculation may force them to pay more attention (or a different kind of attention) to their work that ...
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  • 4,412
21 votes
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Metonymy in mathematics

Metonymy and its relatives, metaphor, polysemy, synecdoche occur all over the place in mathematical writing, and sometimes cause students problems and sometimes don't, because those thought processes ...
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21 votes

A Series of Unfortunate Examples!

Personally, I refer to this phenomenon as students "submarining" a broken understanding on a particular kind of problem. Example #1: Our in-house elementary algebra textbook, in its first edition, ...
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20 votes
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Misuse of parentheses for multiplication

To answer the ultimate question ("Can anybody explain where this writing tradition comes from?"): It's explicitly taught that way by many U.S. instructors and textbooks. Examples: From the otherwise ...
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20 votes
Accepted

How should a student's inefficient calculation be pointed out?

I like your second option the best: ...wait for them to finish the calculation, or even finish the entire exercise, before I casually tell them there was a more natural way to work out that part? ...
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  • 7,696
20 votes

Multiple students writing $y\frac{d}{dx}$ rather than $\frac{d}{dx}y$ -- why?

When a student writes incorrect notation, ask them to read it out loud. I would say something like: Something here doesn't look right, but we can fix it. Could you read this work out loud? I think ...
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  • 19.1k
19 votes

Why do students only see the last term of a sum abbreviated with an ellipsis?

I suspect that the issue is not so much the ellipsis per se but a problem with notation in general, and in particular with the correct use of the equals sign. At the risk of repeating what I wrote in ...
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  • 16.3k
18 votes
Accepted

Students problems with reasoning, not exactly math

You can say that this is "just reasoning", but the truth is that this is a specific application of basic logic, in particular the implication (if/then) relation. I have a colleague with a ...
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18 votes

Misuse of parentheses for multiplication

I disagree that it is "terribly harmful". Do not prevent them from writing $(2)(5)$. Instead prevent them from writing things that are actually wrong. Thinking that $\sin x$ is $\sin$ times $x$ ...
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  • 6,211
18 votes

How to explain that the sums of numerators over sums of denominators isn't the same as the mean of ratios?

One observation is that (sum of numerators) divided by (sum of denominators) is not well defined. For example, let's work with the two ratios $a=\frac01$ and $b=\frac11$. The ratio of the sum of ...
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  • 7,585
17 votes

Is this just a mistake or a more serious misconception?

It seems clear that there is a certain conceptual gap in the student's understanding. My suspicion is that the student is essentially running the following program in his mind: Initialize factorial =...
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16 votes

Near-universal student mistake on $\lim_{x\rightarrow\infty}e^{x+1}/e^x$

First of all, I think that the problem statement can be confusing. use L'Hospital's rule if possible, or if not, explain why it didn't work and evaluate it by some other method It can be ...
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  • 261
16 votes

Why does the widespread erroneous definition of "irrational number" persist without being taught?

I can think of two related reasons: The characterization via the decimal expansion might be perceived more strongly like a property of the number: "This number is irrational, because this number's ...
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  • 7,582
15 votes

A parabolic arc is not semicircular. But students think so

This is pretty natural, I think. People understand things in terms of things that they already know, and while Calc 1/2/3 students should theoretically have a reasonably well developed 'catalog' of ...
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  • 4,850
15 votes

Mnemonics for some properties in mathematics

Recently, a student in my beginning algebra course offered the following to the class, regarding signed number multiplication: Assuming positivity is like love, and negativity is like hate, then... "...
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  • 7,696
14 votes
Accepted

Students using l'Hôpital's Rule on the terms of a series, instead of the Limit Comparison Test

I have also observed students doing this, and then I gave a 'proof' of it, and told a friend I was going to publish it. Then I did a literature search and discovered that it is false. Even though the ...
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14 votes

Whence the "everything is linear" phenomenon, and what can we do about it?

I guess you have already got plenty to read for "Where does it come from?" part of you question. Thus I just shortly introduce my favorite strategy that I use for "What can we do about it?" part, when ...
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  • 4,316
14 votes

Should students be told they're wrong

This is obviously a subjective topic, but here's my take: As an educator, you should see yourself as a resource to your students. You have certain knowledge that they seek to obtain. You should never ...
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