Definition of abs() function

Question

Let $\text{abs}(a)$ denote the absolute value of $a$. Is it true that $\text{abs}(a)\geq{-a}$? I suppose that $\text{abs}(a)>{-a}$, but my math book says the other way. Please help me to understand is it a misprint in my book, or my misunderstanding. Thank you in advance.

Answer 1

Consider the example of $a=0$. Then $\operatorname{abs}(a) = -a$.

Or consider the example of $a = -1$. Then $\operatorname{abs}(a) = -a = 1$. Similarly, $\operatorname{abs}(a) = -a$ whenever $a<0$.

Answer 2

yes, it's correct - if $a\leq 0$, then $|a|=-a$, and the inequality $|a|\geq -a$ holds.

if $a>0$, then $-a<0$, and so $|a|>0>-a$.

either way, the inequality $|a|\geq -a$ holds.

Answer 3

We have

$$\operatorname{abs}(a)=\max(a,-a)=\left\{\begin{array}{cl}a\;&\text{if}\; a\ge0\\-a\;&\text{otherwise}\end{array}\right.$$

Answer 4

The $abs$ function is defined by:

$\forall{x}\in\mathbb{R},\,abs(x)=|x|=\left\{ \begin{array}{lr} x & : x\ge0\\ -x & : x <0 \end{array} \right.$

So $\forall x\in\mathbb{R},\,|x|\ge0$

Let $a\in\mathbb{R}$.

If $a\ge0$ then $|a|=a$ and so $a\le|a|$

If $a<0$ then $|a|=-a>0>a$ and so $a\le|a|$

Now, if $a\ge0$ then $-a\le0\le|a|$

If $a<0$ then $-a=|a|$ and so $-a\le|a|$.