Step by step
The question
Question: find the inverse of f(x) = √(x − 2).
The inverse goes back: what comes out of f goes in, and what went in comes out.
1 Write y =
First we write y for f(x): y = √(x − 2).
2 Swap x and y
Swap x and y: what comes out now goes in.
x = √(y − 2).
3 Get y alone
To remove the square root, we square both sides.
Squaring removes the square root.
Square both sides: (x)² = y − 2.
To remove − 2, we add 2.
We do it on both sides, so they stay equal.
− 2 and + 2 make zero.
Cross them out.
y is alone: y = x² + 2.
This y is the inverse: f⁻¹(x) = x² + 2.
4 The domain of the inverse
The inverse's x's are f's answers.
f(x) is a square root , so it is 0 or more.
Careful!
Without it, the inverse would give answers f never had.
Its x's are only f's answers: 0 or more.
So the inverse only takes x ≥ 0.
5 Draw both
Draw the grid.
Plot points of f: (2, 0), (3, 1) and (6, 2).
Join them: f(x) = √(x − 2), starting at (2, 0).
For the inverse, swap each point: (0, 2), (1, 3) and (2, 6).
Join them: f⁻¹, starting at (0, 2).
The two graphs are mirror images in the line y = x.
6 Check
Let's check: there and back.
f(6) = 2.
Now put 2 into the inverse.
We get back 6.
It works!
Let's look back.
First: write y =.
Then: swap x and y.
Then: get y alone.
Then: the domain of the inverse.
Then: draw both.
Last: check.