Step by step
The question
Question: solve log₂(x) + log₂(x + 2) = 3.
One rule first: what is inside a log must be positive.
There is no log of 0 or of a negative number.
We use it in the check.
1 Make one log
The product rule, from the log rules lesson: plus outside the logs is times inside one log.
So we get one log: log₂(x(x + 2)).
Careful!
Plus between two logs is not plus inside.
It is times inside one log.
2 Write it as a power
A log is a power.
So 2 to the power 3 is what is inside.
So x(x + 2) is 2³.
2³ is 8.
3 Open the brackets
x times each part, then like terms: x² + 2x = 8.
4 Get 0 on one side
To solve it, we want 0 on the right side.
So we take away 8 on both sides.
We do it on both sides, so they stay equal.
Now we have x² + 2x − 8 = 0.
5 Factor
We need two numbers.
They multiply to −8, and they add to 2.
They multiply to a minus number, so one number is plus and one is minus.
They add to a plus number, so the bigger one is plus.
We list the pairs that make 8, smallest first.
We take the smaller from the bigger.
We need 2.
2 and 4 give 2.
Yes!
With the signs: −2 and 4.
So it is (x − 2)(x + 4) = 0.
6 One bracket is 0
Two numbers multiply to 0.
That only happens if one of them is 0.
So one bracket must be 0.
We make each bracket 0.
Two small equations.
In x − 2 = 0, we add 2 on both sides.
In x + 4 = 0, we take away 4 on both sides.
So x = 2 or x = −4.
7 Check
An answer can be false here: a log of a negative number does not exist.
So we check every answer in the question.
Let's check x = −4.
Inside a log we get −4.
There is no log of a negative number.
So x = −4 is a false answer.
Books call it extraneous.
Now x = 2.
The insides are 2 and 4: both positive.
Times them inside one log: log base 2 of 8 is 3.
It works!
Our answers were:
We cross out the false answer.
Only x = 2 is left.
Let's look back.
First: make one log.
Then: write it as a power.
Then: open the brackets.
Then: get 0 on one side.
Then: factor.
Then: one bracket is 0.
Last: check.