Step by step
The question
Question: solve |2x| = |x + 3|.
Both bars give how far from 0.
Two numbers the same distance from 0 are equal, or opposite.
Like 3 and minus 3.
Careful!
The minus is for the whole of x + 3.
Every sign in the bracket changes.
1 Two cases
So 2x = x + 3, or 2x = −(x + 3).
We keep the bracket: the minus is for all of it.
2 Solve the first case
First case: 2x = x + 3.
3 Get x on one side
There is x on both sides.
We want x on one side only.
So we remove the x from the right side.
To remove x, we take away x.
We do it on both sides, so they stay equal.
x and − x make zero.
Cross them out.
2x − x = x.
So x = 3!
4 Solve the second case
Second case: 2x = −(x + 3).
5 Open the brackets
A minus in front of a bracket changes the sign of every part inside.
x becomes −x.
3 becomes −3.
6 Get x on one side
There is x on both sides.
We want x on one side only.
So we remove the −x from the right side.
To remove −x, we add x.
We do it on both sides, so they stay equal.
− x and + x make zero.
Cross them out.
2x + x = 3x.
Now we have 3x = −3.
7 Remove the 3
3x means 3 times x.
We want just x.
To remove times 3, we divide by 3.
We do it on both sides, so they stay equal.
3 divided by 3 is 1.
So just x is left.
−3 divided by 3 is −1.
So x = −1!
8 Write the answers
So x = 3 or x = −1.
9 Check
Let's check: put x = 3 in.
The left bars give 6.
The right bars give 6 too.
The same!
Now put x = −1 in.
The left bars give 2.
The right bars give 2 too.
The same!
Both answers work!
Let's look back.
First: two cases.
Then: solve the first case.
Then: get x on one side.
Then: solve the second case.
Then: open the brackets.
Then: get x on one side.
Then: remove the 3.
Then: write the answers.
Last: check.