Step by step
The question
Question: find the inverse of f(x) = (x + 3)², x ≥ −3.
The inverse goes back: what comes out of f goes in, and what went in comes out.
1 Why x ≥ −3
Without the rule x ≥ −3, two x's give the same answer.
f(−1) = 4...
...and f(−5) = 4 too.
Going back from 4, which one?
There would be two ways back.
With x ≥ −3, only −1 is allowed.
Now each answer has one way back.
2 Write y =
First we write y for f(x): y = (x + 3)².
3 Swap x and y
Swap x and y: what comes out now goes in.
x = (y + 3)².
4 Get y alone
To remove the square, we take the square root of both sides.
y is f's x, so y is at least −3.
So only the plus root.
Careful!
Not plus or minus!
The inverse must give one answer.
y comes from f's x's, which are at least −3: the plus root.
So √(x) = y + 3.
To remove + 3, we take away 3.
We do it on both sides, so they stay equal.
+ 3 and − 3 make zero.
Cross them out.
y is alone: y = √x − 3.
This y is the inverse: f⁻¹(x) = √x − 3.
5 The domain of the inverse
The inverse's x's are f's answers.
f(x) is a square , so it is 0 or more.
So the inverse only takes x ≥ 0.
6 Draw both
Draw the grid.
Plot points of f: (−3, 0), (−2, 1) and (−1, 4).
Join them: f(x) = (x + 3)², starting at (−3, 0).
For the inverse, swap each point: (0, −3), (1, −2) and (4, −1).
Join them: f⁻¹, starting at (0, −3).
The two graphs are mirror images in the line y = x.
7 Check
Let's check: there and back.
f(−1) = 4.
Now put 4 into the inverse.
We get back −1.
It works!
Let's look back.
First: why x ≥ −3.
Then: write y =.
Then: swap x and y.
Then: get y alone.
Then: the domain of the inverse.
Then: draw both.
Last: check.