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I was reading http://www.purplemath.com/modules/orderops2.htm it shows

 = 16 ÷ 2[2] + 1   (**)  ...  = 5  

The general consensus among math people is that "multiplication by juxtaposition" (that is, multiplying by just putting things next to each other, rather than using the "×" sign) indicates that the juxtaposed values must be multiplied together before processing other operations

However when talking to certain people they all have said there is no such thing as this. There is shorthand which uses normal multiplication order and no "multiplication by juxtaposition" and etc.

Is there a "general consensus among math people" or is this simply incorrect?

  • 0
    My issue is when students write it in a non-programming context. I want to point them to a webpage that says "5/7x" is wrong and they should say either 5x/7 or 5/(7x). Right now, I tell them that anyway, but don't have any sort of authoritative backup.2012-09-02

4 Answers 4

30

So, the question is whether $a/bc$ means $(a/b)c$ or $a/(bc)$. And the answer is, DON'T WRITE $a/bc$, because it will only cause confusion. Some people/software/whatever will make one interpretation, some will make the other, neither one has been endorsed by the Dalai Lama or any other great leader. Put in enough parentheses to make your writing foolproof.

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    I'll trade you numerology for a Good Answer badge.2016-05-09
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It's simply incorrect. If it were correct, then $2x^2$ would really mean $(2 \times x)^2 = 2^2 \times x^2 = 4 \times x^2$, but it doesn't; it means $2 \times x^2$.

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    (With apologies for bringing this back from the grave.) No, because exponentiation has a very high precedence (higher than just about anything else), so it would still be $2 \times x^2$2015-06-30
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$a/bc$, which is $a/b*c$ of course means $(a/b)*c$, and that is for the same reason $a-b-c$ means $(a-b)-c$ and not $a-(b-c)$. The reason being that mathematical expressions are meant to be read from left to right when there is no operator which takes precedence.

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This question is more about how we deal with trolling and nuisances.

Read this page: http://knowyourmeme.com/memes/48293

Then maybe we can start a meta article about identifying and dealing with these threads.

Between the math forums that I moderate and otherwise frequent, and dozens of other forums

(a short list is here: http://www.mymathforum.com/viewtopic.php?f=13&t=20148&p=79150#p79150),

I'd guess that thousands of hours have been wasted on this garbage.

  • 0
    This question is actually rather important when writing computer programs that parse mathematics; if they are to implement support for juxtaposition, which seems reasonable, their authors need to know whether multiplication by juxtaposition has a higher precedence than the usual multiplicative operators or not. It’s far from clear which choice is best — and indeed some software has changed its interpretation, so it’s wrong to regard discussion of this topic as “garbage”, even if much time has been spent on it.2015-06-30