Step by step
The question
Question: expand (2x + 1)(x + 1)(x + 3).
Expand means: multiply out all the brackets.
1 Multiply the first two
First the first two brackets: (2x + 1)(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 2x and 1.
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.
2x times x is 2x².
2x times 1 is 2x.
1 times x is x.
1 times 1 is 1.
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.
2x + x is 3x.
It is like 2 apples plus 1 apple: that is 3 apples.
So the expression is 2x² + 3x + 1.
2 Times the third bracket
Now this answer times the third bracket, (x + 3).
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 2x² and 3x and 1.
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.
2x² times x is 2x³.
2x² times 3 is 6x².
3x times x is 3x².
3x times 3 is 9x.
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.
6x² + 3x² is 9x².
It is like 6 apples plus 3 apples: that is 9 apples.
9x + x is 10x.
So the expression is 2x³ + 9x² + 10x + 3.
4 Check
Let's check.
We pick a number: x = 2.
We put it into the start: 75.
Now into our answer.
Both give 75.
Same value, so 2x³ + 9x² + 10x + 3 is right!
Let's look back.
First: multiply the first two.
Then: times the third bracket.
Then: group like terms.
Last: check.