1.
Solving Absolute Value
Equations & Inequalities
A step-by-step guide to Absolute Value Problems
2.
To put it simply.
What does absolute value mean and why
is it important?
There is a technical definition for absolute
value, but you could easily never need it.
For now, you should view the absolute
value of a number as its distance from
zero.
3.
Think of it as a number line…
Let's look at the number line:
The absolute value of x, is the distance of x from zero.
• This is why absolute value is never negative;
• Absolute value only asks "how far?", not "in which
direction?".
• This means not only that | 3 | = 3, because 3 is three
units to the right of zero, but also that | –3 | = 3, because
–3 is three units to the left of zero.
12345
0
0
1
1
2
2
3
3
4
4
5
5
– 1
– 1
– 2
– 2
– 3
– 3
– 4
– 4
– 5
– 5
4.
The symbols
Absolute value has a symbol, actually
two, just like other operations.
The symbols for absolute value are two vertical lines. They are
meant to surround the value that you want to take the absolute
value of, sort of like parenthesis surround the symbols that they
group.
5.
An example.
Here are two simple examples. Say that I
wanted to take the absolute value of -5. I
would write it like this:
-5
This would be read in English as, “The
absolute value of negative 5.”
6.
We got it? Here’s a few more.
|-6| The absolute value of negative 6.
|10|
|y|
|x|
|-y +2|
|0|
The absolute value of 10.
The absolute value of x.
The absolute value of y.
The absolute value of negative y plus 2.
The absolute value of 0.
7.
Absolute value in action.
So what are the answers? What is the
absolute value of negative 5 equal to?
-5 5=
Five!
8.
How it works for all numbers (inputs)
It’s simple. Well, it’s a simple as this:
If an input is positive, it STAYS positive.
If an input is negative, it becomes positive.
If an input is zero, it stay zero.
9.
Got it? Try to apply it.
|-6| = 6.
|10|
|y|
|x|
|-y +2|
|0|
= 10. Note: NOT negative 10. Taking the absolute
value is NOT the same as taking the opposite.
= x. But note, we still don‟t know what x is.
= y. y might be negative, positive, or zero.
This would have to be graphed. Y can be anything
and then we would shift the graph 2 to the right.
= 0. The absolute value of 0 is 0. Period, end
of story.
10.
Stay with me, there’s more.
Ok, we now know what absolute value
does, but if that’s a new concept to you
then practice it well. To reach the level of
the standard we have to move on.
First lets look at a simple equation and
solve it:
x + 10 = 293
-10 = -10
x = 283
Subtract 10 from both sides.
Solution x = 283.
I hope that doesn’t shock anyone. If it does please go back and
review basic algebra. The rest of this will only confuse you if you
don’t.
11.
Now a little thinking.
Let’s add absolute value into this same
equation:
|x + 10| = 293
This should be read: “The absolute value
of x + 10 equals 293.
Now we just saw that 283 is the answer
to this problem and I will tell you that it is
the ONLY solution. That is it is the only
replacement for x that makes the
statement x + 10 = 293 a true statement.
12.
Another story
With absolute value in the equation:
|x + 10| = 293
Let’s think. What if x + 10 came out to be
-293.
Then we would have |-293| = 293.
And that’s a true statement.
13.
Think even harder.
-303 + 10 equals = -293
So if x equaled -303 then the equation
would be true.
There are TWO solutions to the equation
|x + 10| = 293.
In fact there usually are two solutions to
an equation that involves absolute value.
14.
The good and the bad.
And I have good news and bad news.
The good news is that you don’t have to
GUESS every time you encounter an
absolute value problem.
The more good news is that there is a
systematic method for finding both
solutions.
The bad news is that you will have to
learn and memorize this method.
15.
The method
First isolate the absolute value sign on one side:
It has to read, “The absolute value of
something, equals something.”
With our sample problem we’re already good.
Now you have to change the right side of the
equation and get rid of the absolute value signs.
We are going to have two solutions and so we’re
going to have two equations.
|x + 10| = 293
We have:
x + 10 = 293 x + 10 = - 293and:
16.
Seem strange?
That’s right we have:
x + 10 = 293 x + 10 = - 293and:
It may seem strange to change the right
side of the equation to find out what that
the variable is on the LEFT side, but trust
me it works.
Notice that the absolute value signs are
now GONE. These two are easy to solve.
17.
Two worked out solutions
x + 10 = 293
- 10 = - 10
x = 283
x + 10 = -293
- 10 = - 10
x = - 303
18.
Seem strange?
We get two solutions.
x = 283 x = - 303and:
This may seem strange but they both
make the original equation true. Watch…
| x + 10 | = 293
Plug in 293…..
|283 + 10| = 293
| 293 | = 293
293 = 293 true
19.
Now the other one.
| x + 10 | = 293
Plug in -303…
|-303 + 10| = 293
| -293 | = 293
293 = 293 true
See? This one works too.
20.
Let’s review.
Remember. When the absolute value signs
get involved in an equation then you can
expect that there will be TWO solutions
and constructing TWO equations is
necessary to finding these solutions.
Isolate the absolute value on one side of
the equation.
Make two versions of the equation. In one
make the NON-absolute value side
negative, in the other make it positive.
21.
What about > < < >
The standard demands that we also deal
with inequalities.
Inequalities are also mathematical
statements. That is, they SAY something
about the relationship between these
numbers. And just like when a person
says something, what they say may be
true or it may be false.
Inequalities do NOT make the simple
statement that one side is equal to the
other. Inequalities can say one of four
things:
_ _
22.
What can they say?
> Says that the left side is GREATER THAN the
right side.
< Says that the left side is LESSER THAN the
right side.
≥ Says that the left side is GREATER THAN OR
EQUAL to the right side.
≤ Says that the left side is LESSER THAN OR
EQUAL to the right side.
Examples:
1 < 3 Reads: “One is less than three”, a true
statement.
4 > -2 Reads “Four is greater than negative
two”, and is also a true statement.
23.
More examples:
More examples:
◦ 4 ≤ 4 This says, “four is lesser than or equal to four.” a
true statement.
◦ 8 ≥ 8 This says, “eight is greater than or equal to eight.”
a true statement.
Let’s say some FALSE THINGS just for fun:
◦ 8 ≥ 19. This is read: “Eight is greater than or equal to
19.” but 8 is not greater than or equal to 19, so this is
false.
◦ 9 < -10 This is read: “Nine is less than negative ten”.
Negative numbers are inherently less than positive
numbers. This is false.
24.
The new rule:
Inequalities are EASY to solve if you know how to
solve regular equations. There is just one new
rule that you have to remember:
If you multiply or divide by a
negative number your must turn the
inequality sign towards the other
direction. This flips its meaning.
25.
True for all inequalities:
This is true for inequalities
whether there is an absolute
value sign in the inequality or
not.
26.
Absolute value and Inequality
Now before we get into truly tackling an
absolute value inequality we have to talk
about a rather complex behavior that
happen when you combine absolute value
with an inequality sign.
27.
I’m not going to lie to you, this procedure is
pretty tough so listen very carefully, or play this
part of the video over and over until you get this.
Absolute value can best be understood as: “The
distance that something is from zero on a
number line.”
Let’s start with a very simple absolute value
inequality:
0
x > 3
28.
x > 3
Let‟s read this in a way that will help us draw it on a number line.
It says: “Whatever x is, it must be more then 3 spaces away from 0 on a
number line.”
0
So how do we make that happen?
Read it right to get it right.
29.
x < 3
What if this had a lesser than sign, instead of a greater than sign?
Then it would say: “Whatever x is, it must be less than 3 spaces away
from 0 on a number line.”
0
How do we make that happen?
Read it right to get it right.
30.
Please help.
Now, since this is tough to memorize. Try
this little poem or make up one of your
own.
“If the sign is greater than see you later.”
“If the sign is less then you shouldn’t
stress just stay inside and clean up your
mess.”
31.
Action time!
OK, pause the video here and let that set in.
|-3y -8| + 10 > 100
Can you read this now?
It says: “The absolute value of the
quantity negative 3 times y minus 8, plus
10 is greater than 100.”
If you’re still here, we are moving on.
Consider this example inequality:
32.
Isolate the absolute value
Remember that our first task is to get the
absolute value to be on it’s own on one
side of the inequality.
|-3y -8| + 10 > 100
So in this case what needs to be dealt
with?
That’s right!! the + 10.
How do we get rid of a plus 10?
That’s right!! We subtract 10.
33.
Chugging through the first algebra.
|-3y -8| + 10 > 100
- 10 -10
| -3y – 8 | > 90
So far so good. Now we have to break this into
two related inequalities. What was my terrible
poem again?
“If the sign is greater than see you later.”
“If the sign is less then you shouldn‟t stress just
stay inside and clean up your mess.”
34.
|-3y -8| > 90
That means we will have one inequality that just
gets rid of the absolute value signs.
And one that gets rid of the absolute value sign
and:
◦ Flips the inequality symbol to the other direction.
◦ And changes the sign of the right hand side.
-3y -8 > 90 -3y -8 < -90
becomes:
and…
Create two related inequalities
Here we have “GREATER THAN” so we are going
to “See you later man”. There we are going to
send our arrows to the right and left.
35.
-3y -8 > 90
Recall that we use inverse operations to
solve, with the goal being to get y by itself.
Add 8 to both sides to get rid of the -8.
+ 8 > + 8
-3y > 98 Now divide by negative 3 and flip the
in-equality sign as you do so. This
is necessary to find the correct
solution. Do not forget it.
/ -3 > / -3
y < -32.6
Y is less then negative -32.6 is our answer.
Let’s work the first inequality that we created.
36.
-3y -8 < -90
We are not done. Now we work our second
equation to get our second answer.
+ 8 < + 8
-3y < -82 Now divide by negative 3 and flip the
in-equality sign as you do so. This
is necessary to find the correct
solution. Do not forget it.
/ -3 < / -3
y > 27 1/3
Y is greater than 27 1/3 is our answer.
Add 8 to both sides to get rid of the -8.
37.
Graph it.
Graph our solutions
Let‟s read this in a way that will help us draw it on a number line.
It says: “Whatever x is, it must be more then 32 2/3 spaces away from 0 on
a left side of 0 and more that 27 1/3 spaces away from 0 on the right of the
number line.”
0-20 -10 10 3020-30
y < -32 2/3 y > 27 1/3OR
38.
Now let’s check our solutions but remember that
we have to pick a number slightly different then
our solution or boundary numbers.
One of the things that we have said is that our y
for this problem is:
Pick a value < -32 2/3
y < -32 2/3
Y is less then negative 32 2/3.
What number can we pick the will be just a tiny
bit less then negative 32 2/3?
How about……
-33
I like it.
39.
Now let’s check our solutions but remember that
we have to pick a number slightly different then
our solution or boundary numbers.
The other thing that we have said is that our y
for this problem is:
Pick a value > 27 1/3
y > 27 1/3
Y is greater than negative 32 2/3.
What number can we pick the will be just a tiny
greater than negative 27 1/3?
How about……
27.5
I like it.
40.
Check for TRUTH not Equality.
Let’s check
Plug BOTH of these choices in for y and then
work the math to see for sure that you get down
to a TRUE statement.
Remember that they do not need to be EQUAL.
Our solutions do NOT say that they should be
equal.
They should make are statements tell the truth.
If our statement said it would come out less than
then it should come our less then, and if our
statement said that it should come out greater
than then it should come out greater than.
41.
|-3y -8| + 10 > 100
|-3( ) – 8 | + 10 > 100
|-3(-33) – 8 | + 10 > 100
|99 – 8 | + 10 > 100
| 91 | + 10 > 100
91 + 10 > 100
101 > 100
Here is our original.
Put parenthesis in place of Y.
Plug in -33 for Y
Notice I did not remove
the absolute value this
time. I am NOT following
the solution steps that
I showed you earlier here.
I am just running through
the math and checking my
answer.
The absolute value of 91 is 91,
so now the absolute val signs go
away.
42.
|-3y -8| + 10 > 100
|-3( ) – 8 | + 10 > 100
|-3(27.5) – 8 | + 10 > 100
|-82.5 – 8 | + 10 > 100
| -92.5 | + 10 > 100
92.5 + 10 > 100
102.5 > 100
Here is our original.
Put parenthesis in place of Y.
Plug in 27.5 for Y
Notice I did not remove
the absolute value this
time. I am NOT following
the solution steps that
I showed you earlier here.
I am just running through
the math and checking my
answer.
The absolute value of -92.5 is
92.5, so now the absolute val.
signs go away.
43.
Pause and practice, but there’s more.
Excellent.
Thank you for hanging with me.
DO NOT be discouraged if you didn’t catch all of
that in the first go around.
Just re-play.
Ask questions in the comments
Send us emails at math@whaleboneir.com
44.
|-2y -6| + 5 < 100
•We still have to consider a problem
where the left side is less than the right
side.
•Like this:
45.
|-2y -6| + 5 < 100
•Notice that we are dealing with a lesser than
sign here. So we refer back to our limerick: “If
the sign is greater then „See you later.‟ If the
sign is less then just don‟t stress, just stay
inside and clean up the mess.”
46.
0-20 -10 10 3020-30 1 2
•We are going to have a closed in answer
here. The range of values that will make this
true are going to be between one number and
another number but won‟t include those
numbers.
•This is what our diagram might look like:
•And in set notation we might have:
{ -29 < y < 28 }
but these number are just guesses at this point.
47.
•Let‟s work it.
•Step 1: Get the absolute value sign alone on
one side of the inequality:
Pause and practice, but there’s more.
|-2y -6| + 5 < 100
- 5 < - 5
| -2y – 6 | < 95
By subtracting 5 from both
sides.
48.
| -2y – 6 | < 95 Now we create two related
inequalities out of
this, which will allow us to
get rid of the absolute value
signs.
-95 < -2y – 6 and -2y – 6 < 95
-95 < -2y – 6 and -2y – 6 < 95
+ 6 +6 +6 +6
-89 < -2y and -2y < 101
44.5 > y y < -51
{ -51 < y < 44.5 }
49.
Check for TRUTH not Equality.
Let’s check
Plug BOTH of these choices in for y and then
work the math to see for sure that you get down
to a TRUE statement.
Remember that they do not need to be EQUAL.
Our solutions do NOT say that they should be
equal.
They should make are statements tell the truth.
If our statement said it would come out less
than, it should come our less than, and if our
statement said that it should come out greater
than then it should come out greater than.
50.
|-2y -6| + 5 < 100 Our original
|-2( ) -6| + 5 < 100 Carefully put in
parenthesis for y.
|-2(-50) -6| + 5 < 100 We need Y> -51.
So I‟ll chose -50.
|100 - 6| + 5 < 100 -2*-50 = 100
|94| + 5 < 100 100 – 6 = 94
94 + 5 < 100 The ABS of 94 is 94.
99 < 100 A true statement. This
answer works.
51.
|-2y -6| + 5 < 100 Our original
|-2( ) -6| + 5 < 100 Carefully put in
parenthesis for y.
|-2(44) -6| + 5 < 100 We need Y< 44.5 So
I‟ll chose 44.
|-88 - 6| + 5 < 100 -2*-50 = 100
|-94| + 5 < 100 -88 – 6 = 94
94 + 5 < 100 The ABS of -94 is 94.
99 < 100 A true statement. This
answer works.
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