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What is the rationale behind saying that $|x|=1$ implies $x=\pm 1$?

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    Because $+1$ and $-1$ are the only integers whose distance from $0$ is $1$? Note $|1|=1=|-1|$, so $x$ can take either value.2010-12-09
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    *please* don't use the title as an integral part of your message. Make the *body* of your message self-contained, even if that means you have to write the same thing twice, once in the title and once in the body.2010-12-09
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    @yunone: Even better, they are the only real numbers.2010-12-10
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    @Sean, ah yes of course, very true.2010-12-10

5 Answers 5

1

$|1| = 1$ and $|-1| = 1$, and there are no other real numbers $x$ such that $|x|=1$.

7

As in other's answers, the absolute value of $x$ can be defined as the distance between $x$ and 0. When $x$ is a real number, we can think of $x$ as being on a number line, and there are two locations on a number line that are 1 unit away from 0: 1 unit in the positive direction (called 1) and 1 unit in the negative direction (called "-1").

If $x$ can be a complex number, we can think of $x$ as being a point in the plane, and there are a whole lot more locations that are a distance of 1 from 0, such as $\frac{3}{5}+\frac{4}{5}i$ (1 and -1 are also still 1 unit away from 0, so they are still possible values of $x$).

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    the fact that you assume x is real is integral to the argument, thanks for making this point!2010-12-10
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Absolute value of a number is its distance from zero on the number line. The absolute value of a number n is denoted by, |n|.

So here if $|x| = 1$ implies, in the number line the distance from zero on the number is 1.$\text{ Hence, } x = \pm 1$.

3

You should think of the absolute value as being the distance a point is away from 0. So with $|x| = 1$, we want all of the points that are one unit away from 0. We see that 1 and -1 are the only values for x that satisfies this. Therefore, with $|x| = 1$ we have that $x \in\{-1,1\}$.

I hope this helps.

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    Well, to be precise, you should conclude that $x \in \{ -1,1\}$, not that $x$ equals that set. $x$ is a number, not a set.2010-12-09
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    @Fredrik -- You are correct, thank you for catching that.2010-12-14
2

If thinks in this as $f(x)= |x|$, you need to see which are the preimages of $1$.

Graphicaly you could to get this: alt text

Blue : $f(x)=|x|$
Red : $g(x)=1$

Thus, $f(x)=1$ only on the intersection points of these two functions, is say on $x=\pm 1$