Absolute value $\lvert x \rvert >-1$ (precalculus)

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How can $\lvert x \rvert >-1$ be true for all real $x$?

If $x\geq 0$: $x>-1$.

If $x<0$: $-x>-1 \iff x<1$.

So $-1<x<1$.

But if I for instance take $x=-5$ I get $\lvert -5 \rvert >-1 \iff -(-5)>-1 \iff 5>-1$. This is true but contradicts $-1<x<1$.

What is wrong here?

Update:

Would it be any difference if I instead had $\lvert x\rvert \geq -1$?

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There are 5 best solutions below

0
On

Your analysis is flawed. Your conclusion "$x>-1$" on the second line only follows under your condition that "$x\geq 0$". But the conclusion "$x<1$" on the third line only follows under your condition that "$x<0$".

These are mutually exclusive conditions -- there is no $x$ that simultaneously satisfies them both. You are combining conclusions from different assumptions.

2
On

it is true, since for $$|x|$$ we have $$|x|\geq 0$$ and $$0>-1$$

1
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These are two independent cases. A particular value of $x$ satisfies one of the cases, not both. Therefore combining the inequalities is incorrect.

You have "if $x<0$ then $-x > -1$" and "if $x\geq 0$ then $x > -1$" but no number satisfies both of those conditionals.

0
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Don't approach it like that. I mean, don't consider $x \ge 0$ as one case and $x < 0$ as another case. Breaking into two inequalities like you did is fine, but technically they are not "$x \ge 0$" and "$x < 0$".

$|x| > -1$ is really the following compound inequality:

$$x > -1 \text{ OR } x < -(-1)$$

I think you approached it with AND in mind instead of OR.

But since it's "$|x| > \ $" then it should be OR.

The first inequality just gives you $x > -1$. The second one is $x < 1$. So you get:

$$ x > -1 \text{ OR } x < 1$$

And together these two inequalities just mean $x$ can be any real number.

Formal side note: OR is union: $\cup$. AND is intersection: $\cap$.


An easier way to approach this specific problem: $|x| \ge 0$ no matter what $x$ is. And since $0 > -1$, then no matter what $x$ is, you will always have $|x| \ge 0 > -1$. Therefore $|x| > -1$ is true for all real values of $x$.

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Hint:

your mistake is in the logical formulation of the statement. The correct formulation is:

$|x|>-1$ if and only if $\{x\ge 0$ and $x>-1\}$ or $\{x<0$ and $-x>-1\}$

If you solve correctly for the logical connectives you don't met contradictions.