Step by step
The question
Question: find the derivative of f(x) = (5 − x)(x² + 2).
1 Name u and v
Call the first bracket u: 5 − x.
Call the second bracket v: x² + 2.
2 Find u′ and v′
The power rule on u gives u′ = −1.
On v it gives v′ = 2x.
3 Use the rule
The product rule: the derivative of u times v is u′ v plus u v′.
Put them in.
Careful!
Not u′ times v′!
Each part needs the other bracket.
4 Open the brackets
What is in front of a bracket multiplies every part inside.
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
All the x terms go together.
Plain numbers go together.
So f′(x) = −3 x² + 10x − 2.
5 Check
Let's check another way: multiply the brackets out first.
f(x) is − x³ + 5 x² − 2x + 10.
The power rule on that gives −3 x² + 10x − 2.
The same!
Right!
Let's look back.
First: name u and v.
Then: find u′ and v′.
Then: use the rule.
Then: open the brackets.
Last: check.