Step by step
The question
Question: factor 8x³ + 125.
1 Write as cubes
Cubes are a number times itself three times: 2 × 2 × 2 = 8, 3 × 3 × 3 = 27.
8x³ is 2x cubed.
125 is 5 cubed.
So it is a sum of two cubes: a is 2x and b is 5.
2 Use the pattern
Here is the pattern.
The first bracket has the same sign as the question.
In the second bracket the middle sign is the opposite one, and the last sign is always plus.
We put a = 2x and b = 5 into the pattern.
Careful!
It is not (2x + 5)³.
Multiplied out, that has x² and x terms too.
3 Tidy up
2x squared is 4x².
2x times 5 is 10x.
5 squared is 25.
So we get (2x + 5)(4x² − 10x + 25).
The second bracket can't be factored: with this pattern it never can.
4 Check
Let's check: we multiply it back out.
Each part of one bracket times each part of the other.
We get 8x³ + 125 again.
So it is right!
Let's look back.
First: write as cubes.
Then: use the pattern.
Then: tidy up.
Last: check.