Step by step
The question
Question: solve |x − 3| = |2x + 1|.
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 2x + 1.
Every sign in the bracket changes.
1 Two cases
So x − 3 = 2x + 1, or x − 3 = −(2x + 1).
We keep the bracket: the minus is for all of it.
2 Solve the first case
First case: x − 3 = 2x + 1.
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 left 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.
Now we have −3 = x + 1.
Let's swap the two sides, so x is on the left.
4 Remove the + 1
We want x by itself.
The + 1 is in the way.
To remove + 1, we take away 1.
We do it on both sides, so they stay equal.
+ 1 and − 1 make zero.
Cross them out.
−3 − 1 = −4.
So x = −4!
5 Solve the second case
Second case: x − 3 = −(2x + 1).
6 Open the brackets
A minus in front of a bracket changes the sign of every part inside.
2x becomes −2x.
1 becomes −1.
7 Get x on one side
There is x on both sides.
We want x on one side only.
So we remove the −2x from the right side.
To remove −2x, we add 2x.
We do it on both sides, so they stay equal.
− 2x and + 2x make zero.
Cross them out.
x + 2x = 3x.
Now we have 3x − 3 = −1.
We want x by itself.
Two things are stuck to x: times 3 and − 3.
The − 3 is on the outside, so it comes off first.
Then the times 3.
8 Remove the − 3
To remove − 3, we add 3.
We do it on both sides, so they stay equal.
− 3 and + 3 make zero.
Cross them out.
−1 + 3 = 2.
Now we have 3x = 2.
9 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.
So x = 2/3!
10 Write the answers
So x = −4 or x = 2/3.
11 Check
Let's check: put x = −4 in.
The left bars give 7.
The right bars give 7 too.
The same!
Now put x = 2/3 in.
The left bars give 7/3.
The right bars give 7/3 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: remove the + 1.
Then: solve the second case.
Then: open the brackets.
Then: get x on one side.
Then: remove the − 3.
Then: remove the 3.
Then: write the answers.
Last: check.