Step by step
The question
Question: find where f(x) = x² + 4x is increasing and where it is decreasing.
The derivative is the slope.
Where it is positive, the graph goes up.
Where it is negative, the graph goes down.
1 Find f′(x)
Use the power rule on each term.
Now we have f′(x) = 2x + 4.
2 Solve f′(x) = 0
At a turning point the graph is flat for a moment: the slope is 0.
So we solve f′(x) = 0.
Take out the 2.
So x = −2.
Careful!
f′ = 0 alone does not tell max or min.
The signs on each side tell us.
3 Sign table
A sign table.
The numbers −2 cut the line into 2 parts.
Test one number in each part: −3, −1.
f′(−3) is −2: negative.
So f goes down there.
f′(−1) is 2: positive.
So f goes up there.
4 Read the table
f is increasing, going up, where f′ is positive: x > −2.
f is decreasing, going down, where f′ is negative: x < −2.
5 Check
Let's check with the graph.
Down, then up: just like the table.
Right!
Let's look back.
First: find f′(x).
Then: solve f′(x) = 0.
Then: sign table.
Then: read the table.
Last: check.