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Topic: Please remind me why -3^2 = -9
Replies: 26   Last Post: Nov 18, 2012 7:51 PM

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Guy Brandenburg

Posts: 80
Registered: 1/30/05
Re: Please remind me why -3^2 = -9
Posted: Oct 18, 2012 2:38 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply
att1.html (18.4 K)

Well, lets think of a nice way to remember it!

How about this:
Please! escape! No! Medical Doctor! Ambulance Surgeon!

Guy

On Oct 18, 2012, at 14:24, "Collinge, Peter (Mathematics)" <pcollinge@monroecc.edu> wrote:

> Part of why students are confused about ?3^2 is that our standard teaching about order of operations is incomplete. We don?t generally specify where negation fits into the order of operations. In fact, PEMDAS (if not banned) should be taught as P.E.N.MD.AS, with negation inserted after exponentiation and before multiplication/division. Then it would be clear that in -3^2, the exponentiation operation is performed before the negation. (I realize that some would say that negation is just multiplying by -1, so would have the same precedence as any multiplication, but clearly many students don?t see that.) But unfortunately P.E.N.MD.AS just doesn?t make a nice, easy-to-remember mnemonic.
>
> Peter Collinge, Professor
> Department of Mathematics
> Monroe Community College, Rochester NY 14623
>
> E-mail: pcollinge@monroecc.edu
> Voice: (585)292-2943
> <image001.jpg>
>
> From: owner-mathedcc@mathforum.org [mailto:owner-mathedcc@mathforum.org] On Behalf Of Jack Rotman
> Sent: Thursday, October 18, 2012 12:39 PM
> To: Wayne Ford Mackey; Guy Brandenburg; john.peterson20@comcast.net; Philip Mahler
> Cc: mathedcc
> Subject: RE: Please remind me why -3^2 = -9
>
> Phil and all:
>
> The expression ?-3^2? deals with the order of operations; the most advanced operations are always done first unless a grouping symbol ?forces? a lower prior operation to be done first. Since exponentiation is more advanced than the sign of a number, the exponent only applies to one symbol (the 3) when there are no grouping symbols.
>
> Instead of banning this type of problem, I believe that we should ban ?PEMDAS? or anything like it. The use of overly simplistic rules (often stated as a sequence of nouns) discourages learning and encourages memorization. If a large rate of correct answers is the only criteria, just have students use a calculator and train them on use of parentheses. If we are teaching mathematics, we should focus on understanding the priority of operations. What Phil internalized was this understanding; saying ?PEMDAS? does not provide any of the understanding to our students [I normally spend about a tenth of my time in class trying to undo the damage of PEMDAS. Undoing partially correct information is terribly difficult!]
>
> Even if we never showed ?-3^2?, students would still be evaluating x^2 for x=-3; knowing that this means squaring a negative is a part of basic literacy in mathematics.
>
> For those with an interest, I?ve posted some ?anti-PEMDAS? comments on my blog (www.devmathrevival.net ). You can use the search box on the site to find them.
> Jack Rotman
> Professor, Mathematics Department
> Lansing Community College
> (517)483-1079 rotmanj@lcc.edu
> www.devmathrevival.net
>
> From: owner-mathedcc@mathforum.org [mailto:owner-mathedcc@mathforum.org] On Behalf Of Wayne Ford Mackey
> Sent: Thursday, October 18, 2012 12:11 PM
> To: Guy Brandenburg; john.peterson20@comcast.net; Philip Mahler
> Cc: mathedcc
> Subject: RE: Please remind me why -3^2 = -9
>
> It should be read as the opposite of 3 squared. Since 3 squared is 9, the opposite is -9. The "-" sign is used in 3 different ways. In front of a natural number it means negative or minus, in front of anything else it means opposite and between two things it means add the opposite.
>
> wayne
>
> From: owner-mathedcc@mathforum.org [owner-mathedcc@mathforum.org] on behalf of Guy Brandenburg [gfbrandenburg@yahoo.com]
> Sent: Thursday, October 18, 2012 6:04 AM
> To: john.peterson20@comcast.net; Philip Mahler
> Cc: mathedcc
> Subject: Re: Please remind me why -3^2 = -9
>
> It's a convention. In a case like that, one really ought to use parentheses to make the meaning clear, since a lot of people, not just youngsters, will get confused.
>
> If one intends to say (-3)*(-3), then write (-3)^2. If one means - (3)*(3), then write - (3^2).
>
> Writing -3^2 is simply asking for confusion.
>
> Guy Brandenburg, Washington, DC
> http://gfbrandenburg.wordpress.com/
> http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
> ============================
> From: "john.peterson20@comcast.net" <john.peterson20@comcast.net>
> To: Philip Mahler <mahlerp@middlesex.mass.edu>
> Cc: mathedcc <mathedcc@mathforum.org>
> Sent: Thursday, October 18, 2012 6:05 AM
> Subject: Re: Please remind me why -3^2 = -9
>
> Phil,
>
> -3 means -1 x 3, so -3^2 is (-1)(3^2) = (-1)(9) = -9.
>
> John Peterson
>
> From: "Philip Mahler" <mahlerp@middlesex.mass.edu>
> To: "mathedcc" <mathedcc@mathforum.org>
> Sent: Thursday, October 18, 2012 5:37:07 AM
> Subject: Please remind me why -3^2 = -9
>
> I have been teaching a long time, and I know from experience that 50% of students will tell me that ?3^2 = +9 on a test or a final, despite having discussed it a few times in a course.
>
> When I first started teaching I taught calculus and precalc. Piece of cake. Then I started with an Algebra I class and couldn?t connect at all for the first week or so. I was ready to believe I couldn?t teach. I simply could not explain how I got the right answers when evaluating expressions... Then I discovered the order of operations (PEMDAS to some). A definition of the order of operations which I had so internalized that I didn?t know there was a rule for it. Since that discovery I?ve been a wonderful teacher. :-)
>
> So... I must be missing something that so many of my students think ?3^2 is +9. What is the rule I have never discovered?
>
> Full disclosure: I think ?k^2, k a constant, should be banned from mathematics texts and tests. -x^2, x a variable, evaluated for say ?3, absolutely (no pun intended) but not ?3^2.
>
> Phil
>



Date Subject Author
10/18/12
Read Please remind me why -3^2 = -9
Phil Mahler
10/18/12
Read Re: Please remind me why -3^2 = -9
John Peterson
10/18/12
Read Re: Please remind me why -3^2 = -9
Guy Brandenburg
10/18/12
Read RE: Please remind me why -3^2 = -9
Wayne Mackey
10/18/12
Read RE: Please remind me why -3^2 = -9
Phil Mahler
10/18/12
Read RE: Please remind me why -3^2 = -9
RotmanJ
10/18/12
Read Re: Please remind me why -3^2 = -9
Paul Hertzel
10/18/12
Read Re: Please remind me why -3^2 = -9
Phil Mahler
10/18/12
Read Re: Please remind me why -3^2 = -9
Clyde Greeno
10/19/12
Read Re: Please remind me why -3^2 = -9
Clyde Greeno
10/19/12
Read Re: Please remind me why -3^2 = -9
Alain Schremmer
10/19/12
Read RE: Please remind me why -3^2 = -9
Wayne Mackey
10/19/12
Read Re: Please remind me why -3^2 = -9
Alain Schremmer
10/19/12
Read Re: Please remind me why -3^2 = -9
Clyde Greeno
10/19/12
Read Re: Please remind me why -3^2 = -9
Alain Schremmer
10/21/12
Read RE: Please remind me why -3^2 = -9
Wayne Mackey
11/14/12
Read Re: Please remind me why -3^2 = -9
Beth Hentges
11/15/12
Read Re: Please remind me why -3^2 = -9
Clyde Greeno
11/16/12
Read Re: Please remind me why -3^2 = -9
Alain Schremmer
10/18/12
Read RE: Please remind me why -3^2 = -9
Collinge, Peter (Mathematics)
10/18/12
Read Re: Please remind me why -3^2 = -9
Guy Brandenburg
10/18/12
Read Re: Please remind me why -3^2 = -9
Phil Mahler
10/18/12
Read Re: Please remind me why -3^2 = -9
Alain Schremmer
11/18/12
Read Re: Please remind me why -3^2 = -9
EddieC
11/18/12
Read RE: Please remind me why -3^2 = -9
Matthews, George
11/18/12
Read Re: Please remind me why -3^2 = -9
Phil Mahler
11/18/12
Read Re: Please remind me why -3^2 = -9
Alain Schremmer

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