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Possible Duplicate:
What is 48÷2(9+3)?

Sorry if this has already been asked, my searches were fruitless...

There's been a buzz lately about the different results given by two different Casio brand calculators for the same expression. It would seem that there's debate on this issue.

The expression is thus:

6 ${\div}$ 2 (1 + 2)

It would appear that the debate is between the scope of the division operator in the equation. Now, as a software developer, I'm accustomed to well-defined operator precedence. The compiler knows exactly in what order terms will be evaluated. Thus, above, the answer is 9.

However, it would appear that the two calculators differ because there's debate about that division operator regarding whether it divides the entire expression or only the first two terms. Rather, is it:

(6 ${\div}$ 2) (1 + 2)

or is it:

(6) ${\div}$ (2 (1 + 2))

Is there an official answer regarding the operator precedence for mathematics in general, outside the realm of computer programming and compilers?

Martin Hansen
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David
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    No, there is no official anything in mathematics... – Mariano Suárez-Álvarez Mar 30 '12 at 18:43
  • @MarianoSuárez-Alvarez: I wonder what the community's take is on the statement from the referenced article: "Mathematics abhors a contradiction; they cannot both be right." – David Mar 30 '12 at 18:46
  • @ZevChonoles: Fair enough. I struggled for a bit wondering if this is even truly on-topic or constructive for this site. Couldn't hurt to try, I suppose. – David Mar 30 '12 at 18:49
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    @David, they can both be right, with respect to different conventions. And the already several years as a practising mathematician have shown me, everyone has her own conventions. – Mariano Suárez-Álvarez Mar 30 '12 at 18:50
  • @ZevChonoles: Indeed, I have cast my vote! – Aryabhata Mar 30 '12 at 18:50
  • @David: I didn't mean to imply that it's not on-topic and constructive; I just wanted to point out that the issue has been dealt with before. – Zev Chonoles Mar 30 '12 at 18:52
  • To the mods in the room... Would you prefer that I delete this question outright, or leave it to be closed as a duplicate and directed to the original? I'm fine either way. – David Mar 30 '12 at 18:52
  • A calculator that interpreted it in the second way would drive me up the wall. (Then again, I’m not really happy with any non-RPN calculator.) I don’t recommend using the expression, simply the left-to-right convention for operations at the same level of precedence isn’t as general as it ought to be, but I do consider the second interpretation genuinely incorrect in the absence of a stated convention allowing it. – Brian M. Scott Mar 30 '12 at 18:54

2 Answers2

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Essentially you are asking for evaluation of the expression $6/2*3$ - whether it is understood as $6/(2*3) = 1$ or $(6/2)*3 = 9$.

In my understanding, it's a question of the order of operations. The first way of interpretation [i.e. $6/(2*3)$] assumes a silent insertion of the brackets. I do not understand why one would be allowed to make such assumptions -- after all, the parentheses were not specified in the original problem, what right does one have to insert them there?

An equivalent question would be if to evaluate $6-2+3$ one uses $6-(2+3)$ or $(6-2)+3$, with the first way being clearly strange - when inserting brackets we normally would factor out $-1$ to transform to $6-(2-3)$, same answer as $(6-2)+3$.

So in short, we go left to right, not right to left.

Edit: grammar and expressions

gt6989b
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Of course, the "real" answer to this question is that we should just specify and agree upon an order of operations once and for all, and not use ambiguous notation....

But in terms of how we interpret these expressions in practice, I disagree with gt6989b's answer. gt wrote $6/2*3$, which I would certainly interpret as $(6/2)*3$. But the original statement was $6\div 2(1+2)$, which I would interpret as $6\div\{2(1+2)\}$.

My theory as to why I would interpret these in "different" ways is that the symbols and their associated spacings are different. In $6/2*3$, I literally see $6/2$ as being more tightly bound then $2*3$ (because of the spacing); if I were writing the expression I would definitely not use $6/2*3$ if I meant $6/(2*3)$. On the other hand, in $6\div 2(1+2)$, I see $2(1+2)$ as being more tightly bound than $6\div 2$; I would definitely not use $6\div 2(1+2)$ if I meant $(6\div 2)(1+2)$.

Similarly, I automatically read $\log 2x$ as $\log(2x)$ and not $(\log 2)x$; I read $\sin \pi/2$ as $\sin(\pi/2)$ and not $(\sin \pi)/2$.

To be clear, I'm reporting my instinctive reactions to these expressions and then trying to theorize why I react that way; I'm not suggesting that we prescribe order of operations as a function of spacing.

Greg Martin
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