Step by step
The question
Question: solve (x − 3)/(x + 4) > 2.
There is an x at the bottom.
The bottom can never be 0.
1 Get 0 on one side
Careful!
Don't times both sides by x + 4.
It can be negative, and then the sign would turn.
So we get 0 on one side instead.
We take away 2 on both sides.
We do it on both sides.
Why both sides?
The sign compares the two sides.
If we change only one side, the answer can change.
Now we have (x − 3)/(x + 4) − 2 > 0.
2 Make one fraction
To take away, both need the same bottom.
2 is 2 times (x + 4), over (x + 4).
Same bottom, so we put the tops together.
We open the bracket: minus 2 times each part.
Like terms together: the top is −x − 11.
Now we have (−x − 11)/(x + 4) > 0.
3 Find the critical points
A critical point is an x where the side with x is 0, or has no value.
Only there can its sign change.
We make each bracket 0.
The bottom counts too: there the fraction has no value.
In −x − 11 = 0, we add 11 on both sides.
In x + 4 = 0, we take away 4 on both sides.
Now we have −x = 11 and x = −4.
To remove the minus, we times both sides by −1.
The critical points are −11 and −4.
4 Make a sign table
Between the critical points the sign stays the same.
So one test number tells us the sign of each part.
We draw a table.
The critical points −11 and −4 split the number line into 3 parts.
At −11, −x − 11 is 0.
So the fraction is 0 there.
At −4 the bottom is 0.
Dividing by 0 is not allowed: no value, so we write a cross.
Left of −11, we test x = −12.
−x − 11 is 1 and x + 4 is −8.
Plus over minus is minus.
Between −11 and −4, we test x = −10.
−x − 11 is −1 and x + 4 is −6.
Minus over minus is plus.
Right of −4, we test x = 0.
−x − 11 is −11 and x + 4 is 4.
Minus over plus is minus.
5 Read the answer
Greater than 0 means positive.
So we want the parts with a plus.
So −11 < x < −4.
6 Show it on a number line
Books also write the answer like this.
It is called interval notation.
A round bracket means the end is not in, like the open circle.
7 Check
Let's check: a number from the answer, −10, put into the question.
13/6 is greater than 2.
True!
And a number outside the answer, −12.
15/8 is not greater than 2.
False, as it should be: −12 is not in the answer.
Let's look back.
First: get 0 on one side.
Then: make one fraction.
Then: find the critical points.
Then: make a sign table.
Then: read the answer.
Then: show it on a number line.
Last: check.