Section 4-3

Solving and Graphing Absolute Value Equations

The absolute value of a number is its distance from zero on a number line. For instance, 4 and –4 have the same absolute value (4):

So, the absolute value of a positive number is just the number itself, and the absolute value of a negative number is its opposite. The absolute value of 0 is 0. Easy!

The absolute value of x is written |x|. So,

|4| = 4

|–4| = 4

|54221.997| = 54221.997

|(–1/4)| = 1/4

A FEW RULES TO REMEMBER:

The absolute value of a product is the same as the product of the absolute values. For instance:

|(9)(–3)| = |9||–3| = (9)(3) = 27

|(–11)(–10)| = |–11||–10| = (11)(10) = 110

|x³y| = |x³||y|

The same goes for quotients.

|(10)/(–5)| = |10|/|–5| = 10/5 = 2

However, the same thing doesn't always work for addition and subtraction!

|–3 + 7| = |4| = 4, but

|–3| + |7| = 3 + 7 = 10

So be careful!

SOLVING ABSOLUTE VALUE EQUATIONS

To solve an absolute value equation, it is usually easiest to get the absolute value alone on one side. Then remove the absolute value signs and write two equations to be solved separately.

Start by dividing both sides by 3.

Now, split the equation into parts, using the fact that if |a| = b, then either a = b or a = –b.

Now solve both equations.

GRAPHING THE SOLUTIONS OF ABSOLUTE VALUE EQUATIONS AND INEQUALITIES

The graph of the solution of an absolute value equation will usually be two points on the number line. For example, the graph of the solution to the last example problem is:

The graph of the solution of an absolute value inequality will usually look like the graph of a compound inequality:

This can be broken into a compound inequality (be careful about direction!):

The graph is shown below. Note that closed dots are used at the endpoints of the rays, since the inequalities are not "strict" ones.