- #1
ecoo
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How come when you do something like -3^2=-9. Does it mean that the dash is a subtraction symbol and not a negative symbol?
Matterwave said:This is basically convention. When you see something like -3^2 without parenthesis, you take the exponent of the positive number 3 and then leave the negative symbol. Essentially -3^2 is shorthand for -(3^2). We put the parenthesis between the number and the exponent by convention. If you want to get the positive number 9, you would write (-3)^2=9.
Really, this is just convention, like reading an equation from left to right, or PEMDAS.
Mentallic said:And keep in mind that the reason (-3)2=9 is because
[tex](-3)^2=(-1\times 3)^2=(-1)^2\times (3)^2 = (-1)\times(-1)\times 9= 1\times 9[/tex]
ecoo said:Sure, but what I was wondering is that the dash makes the number a negative. Why would you have to put it in parenthesis? The dash is simply showing the number is negative. Why would you put 3^2 in parenthesis then multiply by negative 1? Just as if 3 was positive it would be +3. Isn't the dash just saying the number is negative?
Matterwave said:This is basically convention. When you see something like -3^2 without parenthesis, you take the exponent of the positive number 3 and then leave the negative symbol. Essentially -3^2 is shorthand for -(3^2). We put the parenthesis between the number and the exponent by convention. If you want to get the positive number 9, you would write (-3)^2=9.
Really, this is just convention, like reading an equation from left to right, or PEMDAS.
Matterwave said:It's just the way people decided to do things. There's nothing really wrong if you define -3^2 to equal (-3)^2, but we defined it the other way as -(3^2).
People could be completely pedantic and just demand the usage of parenthesis everywhere. So that 1+2*3 must be written 1+(2*3). But this is a hassle, so we use PEMDAS. This eliminates the need for a lot of parentheses. Exponentiation comes before all other operations (other than parenthesis) so you should read -3^2 as -(3^2).
ecoo said:So a negative number is always -1 multiplied by that number? -3 = -1 * 3, but can't it as well just be 1 * -3? Negative 3 is just another number like positive 3, I thought.
Mentallic said:-3 IS just another number, but each number can be expressed in varying ways, and it turns out that -3 = -1*3 helps us, while -3 and 1*-3 give us no extra insight into why (-3)2=9 and not -9. If you know the rule [itex](ab)^n=a^n\times b^n[/itex] then you can use the fact that [itex]-3=-1\times 3[/itex] to prove to yourself that it is indeed the case.
ecoo said:Yes yes I see that :D but just as you can say 3 = -1 * -3. I forgot about pemdas, but now I see. I just think it makes more sense (to me) to say that -3^2 = 9 instead of -9 (but of course mathematicians follow pemdas, so yea :D)
ecoo said:Yes yes I see that :D but just as you can say 3 = -1 * -3. I forgot about pemdas, but now I see. I just think it makes more sense (to me) to say that -3^2 = 9 instead of -9 (but of course mathematicians follow pemdas, so yea :D)
That is a result of the "precedence" conventions for arithmetic: PEMDAS (or "Please Excuse My Dear Aunt Sally") Any calculations in Parentheses are done first, then exponents, then multiplication and division, then addition and subtraction. Here the exponentiation is done first, 3^2= 9, then the "multiplying" by -1. (It really doesn't matter whether you think of this as "negation" or "subtraction" from 0.)ecoo said:How come when you do something like -3^2=-9. Does it mean that the dash is a subtraction symbol and not a negative symbol?
symbolipoint said:You are going backward from what you began to understand.
3^2 is 9.
+3^2 is 9. The plus sign was not needed.
-3^2 is the OPPOSITE of 3^2; therefore -3^2 is the opposite of 9.
HallsofIvy said:That is a result of the "precedence" conventions for arithmetic: PEMDAS (or "Please Excuse My Dear Aunt Sally") Any calculations in Parentheses are done first, then exponents, then multiplication and division, then addition and subtraction. Here the exponentiation is done first, 3^2= 9, then the "multiplying" by -1. (It really doesn't matter whether you think of this as "negation" or "subtraction" from 0.)
If we used your convention, such that negation has higher priority than exponentiation, then our notation for polynomials starts to fall apart. For example, under your convention,ecoo said:But isn't negatuve 3 just another number like positive ? So they negative sign is just telling that 3 is negatuve. Why would you have to put that in parenthesis?
-3^2 to me should still me -3 * -3
ecoo said:But isn't negatuve 3 just another number like positive ? So they negative sign is just telling that 3 is negatuve. Why would you have to put that in parenthesis?
-3^2 to me should still me -3 * -3
Of course. If you have a number like :##-5## it's actually ##0-5##ecoo said:I get it now after reading this
http://mathforum.org/library/drmath/view/61523.html
So i was right in thinking the dash is actually treated as a subtraction symbol?
ecoo said:But isn't negatuve 3 just another number like positive ? So they negative sign is just telling that 3 is negatuve. Why would you have to put that in parenthesis?
-3^2 to me should still me -3 * -3
gopher_p said:I think maybe where you're getting confused is that you think that the grouping of sybmbols "##-3^2##" has inherent meaning. It does not. It's just a bunch of squiggles on the page until we've given it meaning.
If you believe in alternate universes, then in one of them "##-3^2##" is interpreted as meaning ##(-3)\times(-3)=9##, and this is perfectly reasonable. In this universe, however, we have chosen to interpret "##-3^2##" as meaning ##-(3\times3)=-9##.
Now others here have tried to persuade you by giving you reasons why the notation means what it means. I'm going to tell you that there are no reasons. It's (basically) completely arbitrary. There are two completely reasonable interpretations, and as a group we decided a long time ago to use the second. It's fine if you disagree with that decision; I have a ton of issues with mathematical notation that I think is "wrong". But disagreeing with the chosen notation does not change the fact that, to everyone that "matters", "##-3^2##" means ##-(3\times 3)##.
Now to those here that argue that the notation would get confusing if we chose the alternate interpretation or a different order of operations, understand two things; (1) it only seems confusing to you because you are so used to the commonly accepted conventions and (2) the conventional notation is as confusing to your students (if not more so) than unconventional notation is to you. In a different universe, the distributive law looks like ##c\times a+b=(c\times a)+(c\times b)##, and in another it looks like ##c+(a\times b)=(c+a)\times(c+b)##. This isn't because the math is different in that universe; it's that the symbols used to express the mathematics are interpret differently.
Matterwave said:Although it is true that notational convention is notational convention and is completely arbitrary, if you change only ONE piece of notational convention, without changing the others, you will most likely get unresolvable ambiguities. If the OP wants to change -3^2 to mean (-3)^2, then he has to change basically ALL of our conventions around to make them consistently interpretable. This was what I was trying to get at earlier when I said statements such as 1-3^2 would be ambiguous if we chose to keep PEMDAS. Of course, we can very well discard PEMDAS, and choose 1-3^2=1+(-3)^2 with implicit addition symbols...but...then there's other rules we have to define!
gopher_p said:If you decide that subtraction/negation takes precedence over exponentiation, as is the case with the interpretation ##-3^2=(-3)^2##, then ##1-3^2=(1-3)^2##.
ecoo said:So from what I am thinking, would it be wrong to think that in the equation -3^2 = -9, then the dash in front of the 3 is a subtraction symbol. So if we have subtraction a dash symbol and negative numbers a superscript dash symbol, then the dash in front of 3 would be a regular dash and the dash in front of 9 would be superscript dash.
So it would seem that if the dash in front of 3 were superscript, then the answer would be positive 9.
micromass said:There aren't different types of dash symbols. You only have two types.
1) Substraction. This is a binary operation between two numbers. Like ##3-9## and ##1-1##.
2) Negative of a number. This is a unary operation on one number, like ##-3##.
The two are linked by ##0-x=-x## and ##x-y = x+(-y)##.
So ##-3^2## can be seen as ##0-3^2## if you want, but the result is still ##-9##. The only way you can get this to yield ##9## is by inserting brackets.
ecoo said:Yes yes I have nothing wrong with the answer, but to get that answer the dash must represent subtraction, right? Parenthesis around dash and the number (e.g. (-3)^2) is just to clarify that the dash is the symbol for negative and not subtraction.
micromass said:No. The dash in ##-x## just means "take the negative of ##x##".
Similarly, if we write ##-3^2##, then this means "take the negative of ##3^2##," which is ##-9##.
An exponent on a negative number is a mathematical notation that indicates how many times the negative number should be multiplied by itself. It is written as a superscript after the negative number, such as -23 which means -2 multiplied by itself 3 times.
The rule for exponents on negative numbers is that an even exponent will always result in a positive number, while an odd exponent will always result in a negative number. For example, -24 is equal to 16, but -23 is equal to -8.
To simplify an exponent on a negative number, you can rewrite it as a fraction with a positive exponent. For example, -32 can be rewritten as (-3)2 which is equal to 9.
Yes, an exponent on a negative number can be a decimal or fraction. For example, -20.5 is equal to the square root of -2, and -21/2 is also equal to the square root of -2. However, the result of an exponent on a negative number with a decimal or fraction may not always be a real number.
When you have a negative exponent on a negative number, it can be rewritten as a positive exponent on the reciprocal of the negative number. For example, -2-3 can be rewritten as 1/(-2)3 which is equal to -1/8.