Step by step
The question
Question: solve (x − 1)/(2x + 4) > 0.
There is an x at the bottom.
The bottom can never be 0.
1 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 − 1 = 0, we add 1 on both sides.
In 2x + 4 = 0, we take away 4 on both sides.
Now we have x = 1 and 2x = −4.
To remove times 2, we divide both sides by 2.
The critical points are 1 and −2.
2 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 −2 and 1 split the number line into 3 parts.
At −2 the bottom is 0.
Dividing by 0 is not allowed: no value, so we write a cross.
At 1, x − 1 is 0.
So the fraction is 0 there.
Left of −2, we test x = −3.
x − 1 is −4 and 2x + 4 is −2.
Minus over minus is plus.
Between −2 and 1, we test x = 0.
x − 1 is −1 and 2x + 4 is 4.
Minus over plus is minus.
Right of 1, we test x = 2.
x − 1 is 1 and 2x + 4 is 8.
Plus over plus is plus.
3 Read the answer
Greater than 0 means positive.
So we want the parts with a plus.
So x < −2 or x > 1.
Careful!
At x = −2 the bottom is 0.
Dividing by 0 is not allowed.
So that point is never in the answer.
4 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.
∞ means it goes on for ever.
∪ means or.
5 Check
Let's check: a number from the answer, −3, put into the question.
2 is greater than 0.
True!
And a number outside the answer, 0.
−1/4 is not greater than 0.
False, as it should be: 0 is not in the answer.
Let's look back.
First: find the critical points.
Then: make a sign table.
Then: read the answer.
Then: show it on a number line.
Last: check.