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Re: Please remind me why -3^2 = -9
Posted:
Nov 14, 2012 11:15 PM
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(-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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