Step by step
The question
Question: expand (x − 2)(x − 1)(x + 4).
Expand means: multiply out all the brackets.
1 Multiply the first two
First the first two brackets: (x − 2)(x − 1).
The third waits.
Careful!
We can't just multiply the first parts and the last parts.
Every part meets every part.
Each part of the first bracket multiplies the whole second bracket.
The parts of the first bracket are x and −2.
Each one takes the whole second bracket.
What is in front of a bracket multiplies every part inside.
So we write each part times what is in front.
Watch the signs.
x times x is x².
x times (−1) is −x.
(−2) times x is −2x.
(−2) times (−1) is 2.
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
All the x terms go together.
Each colour is one kind.
−x − 2x is −3x.
So the expression is x² − 3x + 2.
2 Times the third bracket
Now this answer times the third bracket, (x + 4).
Each part of the long bracket meets every part of the third.
Each part of the first bracket multiplies the whole second bracket.
The parts of the first bracket are x² and −3x and 2.
Each one takes the whole second bracket.
What is in front of a bracket multiplies every part inside.
So we write each part times what is in front.
Watch the signs.
x² times x is x³.
x² times 4 is 4x².
(−3x) times x is −3x².
(−3x) times 4 is −12x.
3 Group like terms
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
All the x² terms go together.
Each colour is one kind.
4x² − 3x² is x².
It is like 4 apples take away 3 apples: that is 1 apple.
−12x + 2x is −10x.
So the expression is x³ + x² − 10x + 8.
4 Check
Let's check.
We pick a number: x = 3.
We put it into the start: 14.
Now into our answer.
Both give 14.
Same value, so x³ + x² − 10x + 8 is right!
Let's look back.
First: multiply the first two.
Then: times the third bracket.
Then: group like terms.
Last: check.