Even More on Order of Operations
Date: 10/26/2017 at 16:22:13 From: Steven Subject: Solving 8/4(3-1). PEMDAS vs Distributive I'm curious to know what the answer is for this: 8/4(3 - 1) Following strictly PEMDAS, the answer is 4: 8/4(2) 2*2 4 However, if you follow the distributive property, you get 1: 8/((4*3) - (4*1)) 8/(12 - 4) 8/8 1 Which one would be correct and why? Both are valid, so I'm conflicted as to what would be the correct answer. It should be right or wrong, not two different answers being right.
Date: 10/26/2017 at 17:01:15 From: Doctor Peterson Subject: Re: Solving 8/4(3-1). PEMDAS vs Distributive Hi, Steven. The problem is not a conflict between PEMDAS and distribution; it is that strict interpretation of PEMDAS conflicts with one's natural impression of the meaning of the expression, so that you unknowingly apply an alternative interpretation when you think you are just applying the distributive property. When you distributed, you ASSUMED that it was the 4, not 8/4, that was multiplying the (3 - 1). In doing so, you were bypassing the rules and just doing what felt right. If you followed the rules AND distributed, you would get this: (((8/4)*3) - ((8/4)*1)) ((2*3) - (2*1)) 6 - 2 4 Because it is so natural to see the multiplication as tightly attached to the parenthesis (especially when the division is indicated by an obelus rather than a slash/virgule), some authorities state an extra rule: when a multiplication is indicated only by juxtaposition (no symbol), it is done before divisions. What many people don't realize is that the "rules" we teach are only an attempt at DESCRIBING what mathematicians did for a long time without explicitly stating what rules they were following. They do not PRESCRIBE what inherently must be done, a priori. In just the same way, English grammar came long after English itself, and has sometimes been taught in a way that is inconsistent with actual practice, in an attempt to make the language seem perfectly rational. See this page: History of the Order of Operations http://mathforum.org/library/drmath/view/52582.html In my opinion, the rules as usually taught are not the best possible description of how expressions are evaluated in practice. (This is supported by a recent correspondent who found articles from the early twentieth century arguing that the rules newly being taught in schools misrepresented what mathematicians actually did back then.) Unfortunately, for decades schools have taught PEMDAS as if it must be taken literally, so that one must do all multiplications and divisions from left to right, even when it is entirely unnatural to do so. The better textbooks have avoided such tricky expressions; but others actually drill students in these awkward cases, as if it were important. The added rule about juxtaposition is an attempt to correct this. But the reality is that, since teachers give two different sets of rules that produce different values for expressions like yours, all we can really say is that such an expression SHOULD NEVER BE WRITTEN! In the absence of agreement on the meaning of such an expression, we have to consider it ambiguous unless you first state what rules you are following. And in fact, such expressions are rarely written in practice; beyond the elementary grades, we typically represent most divisions with a horizontal fraction bar. That, I think, is why this "error" has never been corrected: it doesn't matter much. We get questions about this quite often, and they routinely raise other issues. Here is a typical recent reply: This question, and others like it, have been going around on social media for years, always causing this sort of conflict (perhaps even intentionally). We've had several questions about it just this month. It's totally unnecessary. On the whole, this is what you will get if you follow the "rules" as usually taught: 6 / 2 (1 + 2) \_____/ 6 / 2 * 3 \___/ 3 * 3 \______/ 9 Those who say you should distribute first are putting the cart before the horse: you can't apply tricks to evaluate an expression before you first know what it MEANS, but they are thinking that the distributive property affects the meaning. (In fact, the distributive property is a waste of time here, because it makes you do two multiplications where only one is needed!) The meaning is determined by the order of operations. Is the multiplication supposed to be done before or after the division? The trouble is that sources differ on the "rules." The basic "order of operations" as commonly taught says only that multiplication and division are to be done (in the order they appear, left to right) before addition and subtraction, as above. But some authors add an "extra" rule that when multiplication is indicated by mere juxtaposition -- without any explicit symbol, as in "2x" or "2(3)" -- it is done first, taking it as a unit: 6 / 2 (1 + 2) \_____/ 6 / 2 ( 3 ) \_______/ 6 / 6 \_______/ 1 This is what your "opponents" are doing, though without a proper reason; it is also popular among kids who just do what feels right to them. But, as I said, there are also textbook authors and others who explicitly teach it. In books and handwritten math beyond the elementary level, we hardly ever use the horizontal division symbol, but use fraction bars instead, which leaves no ambiguity. As a result, the math community has never had a need to make a choice on this situation! It's essentially been left undefined, and it is textbook authors who came up with explicit "rules" to describe what is really just a language that developed organically, based not on carefully stated rules but on tacit agreement. So which is the "right" way to read such an expression depends on what rules are in force in a particular community (math class, journal, etc.) -- and what was intended by the writer. As a result, in problems such as this, the error is being made primarily not by those who give "wrong" answers, but by those who post the problem in the first place (or pass it on). Anyone who really wants to do math correctly will want to communicate clearly about it, and will avoid anything ambiguous or uncertain. They should either fully parenthesize, or use the horizontal fraction bar, which makes the order clear: 6 6 -------- or ---(2 + 1) 2(2 + 1) 2 Here are some examples of past answers to similar questions: Order of Operations Dispute http://mathforum.org/library/drmath/view/57025.html More on Order of Operations http://mathforum.org/library/drmath/view/57021.html Even some calculators differ on this: Implied Multiplication and TI Calculators http://mathforum.org/library/drmath/view/72166.html Here is an example of someone who teaches the "extra" rule and believes that it is generally accepted: http://www.purplemath.com/modules/orderops2.htm - Doctor Peterson, The Math Forum at NCTM http://mathforum.org/dr.math/http://mathforum.org/library/drmath/view/57021.html
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