53
"Because we said so" is a bit of a conversation closer, I agree. But "Because some people agreed a long time ago to define it that way so we could have conversations where we all understood each other. Does that seem like it would be a good idea?" is both more inclusive and more correct.
I don't even think it's that hairy to talk through the FTA with ...
34
There was a multiplication table posted on the wall. Like this
\begin{alignat}4
1 &\quad 2 &\quad 3 &\quad 4 &\quad\cdots\\
2 &\quad 4 &\quad 6 &\quad 8 &\quad\cdots\\
3&\quad 6 &\quad 9 &\quad 12 &\quad\cdots\\
4&\quad 8 &\quad 12 &\quad 16 &\quad\cdots\\
\vdots&\quad \vdots &\quad \...
27
Why do students have problems with showing that something is well-defined? How can this be improved?
Maybe, your students have a belief problem. They will rarely (maybe never) have encountered problems where something was not well-defined.
If you have never been in trouble since everything you were shown was well-defined, then you don't even understand the problem! (Even harder: after proving that something is well-defined, the world looks right like it ...
20
I think it is a good thing to talk about how there are some concepts where there are choices for where you start when definining them. It happens in linear algebra too, with the definition of linear dependence. You need to talk about how there is this web of connected properties, and it depends on what you're trying to achieve as to where you start. I do ...
17
I think there are serious pedagogical problems with such an approach. Here is a good general rule for explaining any kind of math:
Skipping over the motivation doesn't make something easier to understand.
As math educators, our instinct is always to simplify the math that we are presenting as much as possible. We always want students to understand the ...
16
It's funny you should ask this now, because I just taught a Math Ed graduate class on this topic the other day, so a lot of these thoughts are fresh in my mind.
The pedagogical sequence we conventionally follow with respect to complex numbers -- first introducing them in a "just pretend" way, and only later (for some students) providing a geometric ...
16
Not formal research, but some decades of experience teaching both undergrad and graduate level courses, and "editing" PhD theses and such:
It appears that even many serious professional mathematicians do not understand the difference between a "definitional" iff and an "assertive" iff. This is entirely parallel to an assignment equality versus an assertive ...
16
$2x$ is an expression, a phrase. Compare it to "two ducks". This is neither true nor false. It doesn't have a 'truth value'.
$2x = 4$ is an equation, a statement. Compare it to "two ducks have four legs". This is true (edit: for the ducks, but not necessarily for the $x$).
The meaning of the word "ducks" has not changed. The grammar of what is with that ...
15
Many high school geometry textbooks define an angle as simply
the union of two rays with a common endpoint
The advantage of this definition is its simplicity. Among its disadvantages:
It does not serve well for capturing the idea of a "direction": That is, there is no way to distinguish between a clockwise and a counterclockwise rotation.
It more or ...
15
A linear function is not necessarily a first degree polynomial function: zero function is also linear.
In France the terminology is more appropriate than the traditional English one: a linear function is a function of the form $f(x) = ax$, while a function of the form $f(x) = ax + b$ is called an affine function.
So, strictly speaking, constant functions ...
14
I think a lot depends on context -- what course you are teaching and what the characteristics of that course are. If by "Freshman college calculus" you mean what I think you mean -- namely, a non-rigorous course that treats a lot of things using intuitive and heuristic arguments -- then I wonder if "define" is even the right verb to use. Students come into ...
14
Why do students have problems with showing that something is well-defined? How can this be improved?
I've never tried this in a classroom, but I suspect a lot of the trouble with functions is that students haven't been taught the vocabulary to deal with things that are weaker than functions. For example, they are trying to define a function $f: A \to B$. What they have written down probably defines SOMETHING: Perhaps a subset $R$ of $A \times B'$ for some $...
13
This is a very difficult question to answer; I recommend as a first place to look:
Usiskin, Z. (1988). Conceptions of school algebra and uses of variables. The ideas of algebra, K-12, 8, 19. Link (no paywall).
As Usiskin writes (emphasis in original):
My thesis is that the purposes we have for teaching algebra, the conceptions we have of the subject, and ...
13
I'm going to rewrite this answer to clarify what I think the issue is. I think the OP is imagining a different definition of the ring $k[x]$ than most answerers are. Here are two reasonable definitions:
$k[x]$ is the ring of formal expressions of the form $\sum_{j=0}^{\infty} p_j x^j$ with $p_j \in k$ and we require that $p_j$ is $0$ for $j$ sufficiently ...
12
The situation does indeed change your interpretation of the terminology.
Volume is a general concept of the amount of 3D space something takes up.
For a solid object like a brick, or for a liquid there is no ambiguity and you can simply do a calculation (like $V = \pi r^2 h$ for a solid cylinder), or possibly compare its mass to the mass of a known volume ...
12
While I haven't done a systematic survey, my impression is that the overwhelming majority of pre-calculus and calculus texts define a linear function to be one of the form $f(x) = mx + b$ with no stipulation that $m \neq 0$. Thus, if you define linear to be a polynomial of degree 1 you are likely to be contradicting whatever textbook you are using.
From a ...
11
Turning my comment into an answer as per request:
I think the graphical approach gets the idea across. You could represent $3\times 1$ by a row of three dots; likewise, $3\times 2$ is two rows of three dots; etc. If you asked them to guess what $3\times 0$ is, some would probably tell you you'd have no rows so no dots, so the answer is zero. Humans are very ...
11
I think this is a great question because it is really about the ambiguous/confusing way we use the word polynomial. So I want to use the terms "polynomial" and "polynomial function" separately here.
A polynomial is an algebraic expression. It consists of variables and coefficients and only uses the operations of addition, subtraction, multiplication, and ...
11
A US specific answer: The Common Core State Standards define $\frac{1}{b}$ by saying it is one of $b$ equal parts making up a whole $1$. $\frac{a}{b}$ is then defined as $a$ of these. Connecting $\frac{a}{b}$ to $a \div b$ requires some reasoning.
For instance $\frac{5}{3}$ of a candy bar means you take your one candy bar, divide it into 3 equal sized ...
10
You could reply something like this:
Yes, associating each subset of the integers with its size would be an example of a function. And, in fact this does not just work for the integers but for all the subsets of any set. But note that to do it really for all sets would cause some problems as "the set of all sets" is a problematic notion. However, this ...
10
Why do students have problems with showing that something is well-defined? How can this be improved?
I think that you hit the nail on the head, when you said some are not even aware of what well-defined means. As Anschewski suggests the problem may be that students have not encountered enough non-well-defined operations to fully appreciate the problem.
This Spring I was teaching freshman algebra, and while explaining equivalence relations (prior to getting ...
10
Have a look at the paper written by Nunez et all:
EMBODIED COGNITION AS GROUNDING FOR SITUATEDNESS AND CONTEXT IN MATHEMATICS EDUCATION.
In essence, they argue that it is better to be causious if you want to "motivate the formal definition of continuity starting from the intuition" you have suggested in your question. In the following passage, "natural ...
10
At the secondary level, students have not yet mastered formal mathematics and most will need to continue learning concepts before definitions in many cases. The van Hieles (the Dutch educators who developed the Van Hiele Model of how students learn geometry) insisted that definitions should never come first. They claimed that students should first have ...
10
FYI: here's some pro and con: http://primefan.tripod.com/Prime1ProCon.html
One was originally considered prime. It is prime with the most convenient ("natural") definition. It got excluded from prime-ness because many other higher theorems would be complicated by leaving it as prime. Essentially "prime" -> "prime*". The definition of primeness was ...
9
When I teach calculus I, my initial working definition for $e$ is that it is the real number implicitly defined by:
$$ \lim_{h \rightarrow 0} \frac{e^h-1}{h}=1 $$
Naturally, this allows me to derive that $\frac{d}{dx} e^x = e^x$ as
$$ \lim_{h \rightarrow 0} \frac{e^{x}e^h-e^x}{h}=e^x\lim_{h \rightarrow 0} \frac{e^{h}-1}{h}=e^x. $$
Of course, we know this. At ...
9
I have two intuitions to offer:
A sequence $(a_n)$ may have cluster points (these are points such that every neighborhood contains infinitely many elements of the sequence, or, more precisely, for every neighborhood and every index $N$, there is an element $a_n$ with an index $n>N$ which is in this neighborhood). The $\limsup$ is the largest of these ...
8
This grew a bit long for a comment.
(My first note is similar to Alexander Woo's remark about factoring polynomials; perhaps he intended "polynomials" to subsume the case here, in which we add constant functions...)
Given $413 + 91$, it may not be clear that this can be re-written as $7 \times 59 + 7 \times 13 = 7(59+13)$.
(Plenty of people seem to ...
8
One of my most vivid memories of graduate school was working on a problem concerning the Gelfand-Kirillov dimension of certain rings. I had been puzzling over the definition (which was rather new to me) and had reached the conclusion that for a certain class of rings the GK-dimension was greater than 1. My advisor told me that I could not possibly be ...
8
How should one talk about the question of 1 or 0 being prime ... with primary or middle school children?
Depending on what you did before you will have an easy or a hard task:
If the children were told:
A prime number is a natural number which cannot be divided by other numbers than by 1 and by itself.
... you will have problems explaining why 1 is not ...
7
A textbook I have used first defines $$\ln x =\int_1^x \frac 1 t \, \mathrm{d}t,$$ and then defines the number $e$ as in your Definition a).
Next, the exponential function $e^x$ is defined as the inverse of the function $\ln x$. (They show that this definition of $e^x$ coincides with the usual one from repeated multiplication when $x$ is an integer.)
...
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