47
It's purely a matter of how we choose to define the notation. The main reason for it is that it lets us write polynomial expressions (which are extremely common) without parentheses, e.g., $x^3 + 3x^2 y - 41x + 2z$ rather than $(x^3) + (3(x^2)y) - (41x) + (2z)$.
However, what really matters is that the notation is clear and unambiguous, so expressions like $...
29
A colleague of mine includes on all his tests a line that reads (something like) "you will be graded on what you actually write down, not what I think you may have meant by what you wrote." He spends time carefully discussing what this means (examples like the one you gave are among them) before and after testing times. Normally after the second round (of ...
27
The different notations are to a considerable extent outgrowth of slightly different ways of looking at "the derivative." To me these different ways is the important thing to discuss, the notation is a by product. And, I think it is quite useful to spend some time on discussing these different ways and to mention the different notations in that context. (...
27
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 are "needed" because $25$ means the number twenty-five, the parenthesis are purely for grouping.
"No operator means multiplication" is an extremely common ...
24
I think that, depending on the maturity level of the students, you could just talk to them about why writing $\frac{1}{3}x$ rather than $1/3x$ makes it much clearer what you mean. They should understand.
Remind them that the purpose of writing anything, including mathematics, is to clearly convey an idea to the reader, and the notation $1/3x$ is ambiguous: ...
23
To use $$f'(x)=\lim_{dx\to0}\frac{f(x+dx)-f(x)}{dx}$$ is mathematically correct if $dx$ is the name for a real variable. (If it should be something else it needs to be made clear what it should be.)
It would also be correct to say
$$f'(x)=\lim_{\text{small}\to 0}\frac{f(x+\text{small})-f(x)}{\text{small}}$$ with the understanding that $\text{small}$ is the ...
23
Start by talking about functions in general, not only about functions that can be expressed by a simple formula in x and y. Examples:
The function that maps every non-empty list to its first element.
The function that maps every finite set to its size.
The function that maps color names to RGB triples.
The function that maps days to sunrise times at a ...
22
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, other students, etc. I run questions on it in weekly quizzes; and if I had my druthers, it would be a major component of all tests (in addition to application-...
21
Consider the following observations:
People shorten things with increasing frequency of usage. For example, the most frequent words are short. This is a way of making communication more effective, e.g. see Huffman coding. For example this list has its first 5-letter word at 39th place (first three are for comparison):
$$\begin{array}{c|c|c|c}
\textbf{place} ...
21
You might remind them that $y$ is just a name for a number. When they draw a plot, they draw a bunch of points: maybe $y=3$ here, $y=5$ there, and $y=-2$ over there. But at some point (no pun intended) we want to talk about the entire shape: we want to say that $f$ is symmetric, that $f$ is concave, that $f$ has an asymptote. We can't do that with $y$; ...
19
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 excellent Martin-Gay Prealgebra & Introductory Algebra (sec 1.5):
The × is called a multiplication sign... The symbols ∙ and () can also
be used to ...
19
This is also borderline not-an-answer, but it might be a nice broadening of your students' worldview to know that the "$m$" and the "$b$" are not universally accepted. Showing them this map (even though I do not know its original source, so it may not be accurate in its details) could help their thinking out a bit:
Substantial edit: I now no longer believe ...
18
It was mentioned in other answers that having a more sloppy notation is better to not complicate the communication. This is okay for people who had really understood the concepts of mathematics.
I have the feeling that understanding what a function is one of the those things students don't really understand. Common mistakes (and in my opinion all of them ...
18
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 are basic to our understanding of everything.
Where mathematics comes from, by George Lakoff and Rafael E. Núñez. Basic Books, 2000.
Exploring the role of ...
18
You should tell them these two main benefits:
(1) Function notation is concise! For example, instead of writing "Find $y$ when $x=5$" one can simply write "Find $f(5)$" This becomes very appreciable when dealing with long or complicated problems asking for a lot of information. We also shorten things like this all the time. For instance, instead of writing $...
17
To distinguish not too strictly between $f$ and $f(x)$ allows to operate more easily with functions built up from other functions.
For example, one might want to say things like:
Let us consider the function $\sin (3 x^2)$ on the intervall $[0,1]$.
This can be considered as sloppy in a formal sense, but I think it is still clear and in some contexts ...
16
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 $x$ can happen (and does happen) even without parentheses.
I agree $(2)(5)$ looks strange, but if they can write $(4-2)(4+1)$, what is wrong with $(2)(5)$?
15
While there may be legitimate reasons behind the convention
In $a \times b $ the $a$ denotes the number of terms and the $b$ denotes the individual terms
the larger issue is the mismatch between the teacher's enforcement of that convention and the expressly stated purpose of the formative assessment, which is written at the top of the very same page:
...
15
The crucial thing the students need to realise is that the (e.g.) $x$ that turns up in the function definition is a bound variable. That's what allows it to be freely renamed or indeed omitted without changing the semantics.
Unfortunately, education tends to completely obscure this facet by a) always using the same dumb variable names as if there were a ...
14
It is interesting to see how computer algebra systems deal with this kind of thing. In Maple, for example, you can do the following:
Define f := x^3
Enter diff(f,x) to get the derivative
Enter subs(x=3,f) to evaluate.
Alternatively:
Define g := (x) -> x^3
Enter D(g) to get the derivative
Enter g(3) to evaluate
You can convert between the two idioms ...
14
One reason is that mathematics was not handed down by the gods fully formed and unambiguous. It is a human construction over a very long time and mathematical notation even more so.
Any time a notation doesn't "work" for all possible contexts, it's an opportunity for us to talk about this human side of mathematics, and about the pros and cons of notation ...
14
This example strikes me as resembling a phenomenon I've seen where a problem is of the form
Given A, find C
where the main approach to actually solving the problem is:
Using A, obtain B
Using B, obtain C
however, the student seems entirely incapable of even conceiving of doing these as separate steps. Any attempt to help them obtain B just ...
13
There is a certain benefit to "confusing" students; I alluded to the ideas of disequilibrium and the resulting equilibration in an earlier MESE post. More comments about Piaget can be found on this site.
In the context about which you are asking: I think that if you want to introduce multiple notations, then you should, at least, abide by two principles:
...
13
Though I also like to use the notation "the function $x^3$" when appropriate, sometimes this causes problems. For example when I teach inverse functions, it is difficult to grasp that the $x$ in $x^3$ is not the same $x$ as in $\sqrt[3]{x}$.
I always have problems with first year students who are used to the notation that $x$ is in the domain and $y$ is in ...
13
A reason why this form might be preferred is the way one says it:
$5 \times 3$ is read out "five times three" so it says take $3$ five times, hence it "is" $3+ 3+ 3 + 3 + 3$.
However I doubt there is any real standard. For what it's worth Wikipedia disagrees with itself.
On the page on Multiplication it has $a \times b$ as $b + \dots + b$.
On the page ...
13
I wish to give a slightly different answer compared to the others.
Strict and Standardized Notations is Very Important
They not only help us communicate better, they also help us think. They prime us to remember things and understand things better. For example, if I see $a^2 + b^2 = c^2$, I think Pythagoras Theorem and right angle triangles. If I see $k^2 +...
13
Just because you've defined a meaning for $a < b < c$ does not mean that any mishmash of other relational operators becomes equally well-defined as notation. Stick with what you've defined for a chained equality and don't permit arbitrary, nonstandard, off-track jaunts.
For example, here's the treatment in Sullivan, College Algebra, Sec. 1.5. Note ...
13
Because x and y are just variable names
It happens that sometimes y=f(x), but other times z=f(x,y), w=f(x,y,z), or x=f(y) for that matter. All of these variable names are syntactically equivalent, and the mere existence of "x" and "y" in an equation does not necessarily connote that "x" is the independent variable and "y" the dependent. Thinking of the ...
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