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

 Messages: [ Previous | Next ]
 Beth Hentges Posts: 105 Registered: 10/16/06
Re: Please remind me why -3^2 = -9
Posted: Nov 14, 2012 11:15 PM

(-3)^2 is clear

- -(3^2) is clear

We have to choose which we mean when we write the following.

- -3^2

The reason we choose -3^2 to mean -(3^2) is because we do powers before multiplication and because -a = -1*a.

(-3)^2 = (-1*3)^2 , and the parentheses tell us to do the multiplication first.

- -(3^2) = -1*(3^2), and the parentheses tell us to do the power first.

- -3^2 = -1*3^2, and we do powers before multiplication.

As for PEMDAS, I use PEMA. Otherwise, students think PEMDAS says to do multiplication before division. Also, when I write it in words, for the E for exponents (which really should be another P for powers), I write, "Do exponents and roots from left to right." So, even if I did use PEMDAS it would be PERMDAS. Then, we also have to be careful with absolute value as well.

Beth in MN
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Date Subject Author
10/18/12 Phil Mahler
10/18/12 John Peterson
10/18/12 Guy Brandenburg
10/18/12 Wayne Mackey
10/18/12 Phil Mahler
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10/18/12 Paul Hertzel
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10/19/12 Wayne Mackey
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10/19/12 Clyde Greeno
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10/21/12 Wayne Mackey
11/14/12 Beth Hentges
11/15/12 Clyde Greeno
11/16/12 Alain Schremmer
10/18/12 Collinge, Peter (Mathematics)
10/18/12 Guy Brandenburg
10/18/12 Phil Mahler
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11/18/12 EddieC
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11/18/12 Phil Mahler
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