On the number axis, the distance from the point to the origin of a number is called the absolute value of this number, and the absolute value is represented by "| |". On the number axis, assuming a> b, and a> 0, b> 0, then represent the value of the distance between the point of the number a to the point of the number b, read as the absolute value of a-b, denoted as | a-b |.

For example: | 5 | refers to the number axis represents the number of 5's point and the origin distance, this distance is 5, so the absolute value of 5's is 5. Similarly, | -5 | refers to the number on the number axis represents the origin, the distance is 5, so the absolute value of-5's is also 5.

The absolute value of non-negative number positive and 0 is itself, and the absolute value of non-positive number negative and 0 is its opposite number.

a is indicated as '| a |'. Read as "the absolute value of a".

The absolute value of the real number a is always non-negative, namely | a | 0.

The absolute values of two numbers with opposite numbers are equal, i. e., | -a | = | a | (because they are equally distant from the origin on the number axis).

If a is a positive number, then satisfy the x of | x | =a has two values ± a, like | x | =3, then x= ± 3.