Absolute Value in Algebra
Absolute Value
means ...
... only how far a number is from zero:
 |
|
"6" is 6 away from zero,
and "-6" is also 6 away from zero.
So the absolute value of 6 is 6,
and the absolute value of -6 is also 6 |
Absolute Value Symbol
To show that you want the absolute value of something, you put "|" marks either side (called "bars"), like these examples:
 |
The "|" can be found just above the enter key on most keyboards. |
More Formal
So, when a number is positive, we leave it alone, when it is negative we change it to positive, and when it is 0 we leave it at 0.
This can all be written like this:
This says: the absolute value of x equals:
- x when x is greater than zero
- 0 when x equals 0
- -x when x is less than zero (this "flips" the number back to positive)
Here is an example:
Example: what is |-17| ?
Well, it is less than zero, so we need to calculate "-x":
- ( -17 ) = 17
(Because two minuses make a plus)
Useful Properties
Here are some properties of absolute values that can be useful:
|a| ≥ 0 always!
That makes sense ... |a| can never be less than zero.
|a| = √(a2)
Squaring a makes it positive or zero (for a as a Real Number). Then taking the square root will "undo" the squaring, but leave it positive or zero.
|a·b| = |a|·|b|
Means these are the same:
- the absolute value of (a times b), and
- (the absolute value of a) times (the absolute value of b).
Which can also be useful when solving
|u| = a is the same as u = ±a and vice versa
Which is often the key to solving most absolute value questions.
Example: solve |x+2|=5
Using "|u| = a is the same as u = ±a":
this: |x+2|=5
is the same as this: x+2 = ±5
Which will have two solutions:
| x+2 = +5 |
x+2 = -5 |
| x = 3 |
x = -7 |
Graphically
Let us graph that example:
|x+2| = 5
It is easier to graph if you have an "=0" equation, so subtract 5 from both sides:
|x+2| - 5 = 0
And here is the plot of |x+2|-5, but just for fun let's make the graph by shifting it around:
 |
| Start with |x| |
then shift it left to make it |x+2| |
then shift it down to make it |x+2|-5 |
And you can see the two solutions: -7 or +3.
Absolute Value Inequalities
Mixing Absolute Values and Inequalites needs a little care!
There are 4 inequalities:
| < |
≤ |
|
> |
≥ |
| less than |
less than
or equal to |
|
greater than |
greater than
or equal to |
Less Than, Less Than or Equal To
With "<" and "≤" you get one interval centered on zero:
Example: Solve |x| < 3
This means the distance from x to zero must be less than 3:
Everything in between (but not including) -3 and 3
It can be rewritten as:
-3 < x < 3
And as an interval it can be written as: (-3, 3)
The same thing works for "Less Than or Equal To":
Example: Solve |x| ≤ 3
Everything in between and including -3 and 3
It can be rewritten as:
-3 ≤ x ≤ 3
And as an interval it can be written as: [-3, 3]
How about a bigger example?
Example: Solve |3x-6| ≤ 12
Rewrite it as:
-12 ≤ 3x-6 ≤ 12
Add 6:
-6 ≤ 3x ≤ 18
Lastly, multiply by (1/3). Because you are multiplying by a positive number, the inequalities will not change:
-2 ≤ x ≤ 6
Done!
And as an interval it can be written as: [-2, 6]
Greater Than, Greater Than or Equal To
This is different ... you get two separate intervals:
Example: Solve |x| > 3
It looks like this:
Up to -3 or from 3 onwards
It can be rewritten as
x < -3 or x > 3
As an interval it can be written as: (-∞, -3) U (3, +∞)
Careful! Do not write it as
-3 > x > 3 
"x" cannot be less than -3 and greater than 3 at the same time
It is really:
x < -3 or x > 3 
"x" is less than -3 or greater than 3
The same thing works for "Greater Than or Equal To":
Example: Solve |x| ≥ 3
Can be rewritten as
x ≤ -3 or x ≥ 3
As an interval it can be written as: (-∞, -3] U [3, +∞)
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