Step by step
The question
Question: expand (x − 2)³.
Careful!
A power of a bracket is not the power of each part.
(x − 2) to the power 3 is the bracket times itself 3 times.
There are middle terms too.
1 Use Pascal's triangle
Pascal's triangle starts with 1.
Every row starts and ends with 1.
Each other number is the two numbers above it added.
2 The pattern
There are 4 terms.
The numbers in front come from row 3: 1, 3, 3, 1.
The powers of x go down from 3 to 0.
The powers of −2 go up from 0 to 3.
Remember: any number to the power 0 is 1.
3 Work out each term
The first term: 1, x to the power 3, −2 to the power 0.
That is x³.
Second term: 3 times −2 is −6.
So −6x².
Third term: (−2) to the power 2 is 4.
3 times 4 is 12.
So 12x.
Fourth term: (−2) to the power 3 is −8.
So −8.
4 Add them up
So (x − 2)³ is x³ − 6x² + 12x − 8.
5 Check
Let's check with x = 1.
The question: (−1) to the power 3 is −1.
Our answer with x = 1: the terms add up to −1 too.
It works!
Let's look back.
First: use Pascal's triangle.
Then: the pattern.
Then: work out each term.
Then: add them up.
Last: check.