Video Transcript
In this video, we’re gonna look at
some functions and their graphs. And we’re gonna talk about the
concept of a function and its inverse. And we’ll also look at the
algebraic steps to find the inverse of a function and try out this technique on a
few typical questions.
First, let’s look at the function
𝑓 of 𝑥 is equal to a half 𝑥 plus three. And to work with this function,
first of all we input an 𝑥-value. Then the function tells us to half
that number and add three to the result. And out pops the answer. And if we were to plot the graph of
𝑦 equals 𝑓 of 𝑥, it would look like this. Where the line represents the
mapping of all possible input values onto their corresponding output values. And we can use this graph to work
out the 𝑦-values from a given 𝑥-value as well. So if we were given an input value,
say four, we could trace it up to the line and then across to the 𝑦-axis to get the
corresponding 𝑦-coordinate or function output value. So if we’re given a value of 𝑥, we
can use the function to find the corresponding 𝑦-value, either on the graph or
using the equation 𝑦 equals a half 𝑥 plus three.
Now, it’s important to remember
that in this particular function, any value, any real value of 𝑥 is valid for
input. The domain or the set of numbers
which you can input into this function is all of the real numbers. And because that line goes off
infinitely in this direction and in this direction. The range or set of possible
outputs is also the complete set of real numbers on the 𝑦-axis. Now we’ll come back to those two
things, the range and domain, later on in this video.
Going back to the function we were
looking at, what if we were given a 𝑦-value and we wanted to find the corresponding
𝑥- or input value? Well, we could read backwards on
the graph. So we can look at our value on the
𝑦-axis. We can go across to the line and
then down and simply read off the corresponding 𝑥-coordinate. But it would be great to have a
nice equation that we could use without having to draw the graph. Well, of course, we can use our
algebra skills to rearrange the original equation and make 𝑥 the subject. So if 𝑦 is equal to a half 𝑥 plus
three, I could subtract three from both sides to get 𝑦 minus three equals a half 𝑥
plus three minus three. Well, those two things cancel
out. So that’s 𝑦 minus three is equal
to half 𝑥. Then I can multiply both sides by
two to give me two 𝑦 minus six is equal to 𝑥 or 𝑥 is equal to two 𝑦 minus
six.
And we can simply substitute values
in for 𝑦. Do the calculation. And find the corresponding value of
𝑥. So this process and these ideas are
the basic foundation of inverse functions. Now, we saw how the graph mapped
all of the input values from the domain into the corresponding output values in the
range. Well, the inverse function maps all
of the original range values back to the original domain values. Now, if we could flip over the 𝑥-
and 𝑦-axes, we could define a new function which maps what was the range onto what
was the domain. This is how we find the inverse
function. It’s kind of the reverse of the
original function.
So here’s the graph of the original
function. Now let’s add the line 𝑦 equals
𝑥. Then we can reflect everything in
the line 𝑦 equals 𝑥. Which effectively swaps over the
axes and gives us a new function 𝑦 equals the inverse of the original function. So the function and its inverse are
mirror images of each other in the line 𝑦 equals 𝑥. So notice how two maps to four on
the original function and four maps back to two on the inverse function. That’s what inverse functions are
all about.
And briefly going back to the
domain and range. What was the domain for the
original function has now been swapped over onto the 𝑦-axis for the inverse
function. So the domain from the original
function becomes the range of the inverse function. And what was the range for the
original function becomes the domain for the inverse function. Right, let’s go through a few
examples and just talk this through. So first, let’s just finish off
this example that we’ve been looking at.
Find the inverse function for 𝑓 of
𝑥 equals a half 𝑥 plus three.
Now, probably, the easiest way to
approach these questions is to swap 𝑥 and 𝑦 in the equation. And then rearrange to make the new
𝑦 the subject. So swapping over the 𝑥’s and 𝑦’s,
we’ve now got 𝑥 is equal to a half 𝑦 plus three. And just like we did before, we can
now unpick that. So subtracting three from both
sides, we’ve got 𝑥 minus three is equal to a half 𝑦. And then doubling both sides, we’ve
got two 𝑥 minus six is equal to 𝑦. And that gives us our inverse
function. This is the function that maps all
of the old outputs back to their original inputs. It’s the equation of this line
here, our inverse function line.
Now, find the inverse function
given that 𝑓 of 𝑥 is equal to three 𝑥 minus two.
So write 𝑦 equals three 𝑥 minus
two, swap 𝑥 and 𝑦 in the equation, and then rearrange to make 𝑦 the subject. And that gives us our inverse
function.
So both the examples we’ve looked
at so far have had domains and ranges which have included all the real numbers. So they’ve been relatively easy
questions to do. But when you see slightly more
complicated questions, sometimes you have to think carefully about the domain and
the range.
Find the inverse function given
that 𝑓 of 𝑥 is equal to three plus the cube root of 𝑥.
Once you start seeing things like
cube root and square root, then you have to be aware. But, in fact, with cube root,
again, a domain and a range contain all of the real numbers. So we don’t have to worry. So we write out our equation, 𝑦
equals three plus the cube root of 𝑥, swap the 𝑥- and 𝑦-variables, and then
rearrange to make 𝑦 the subject. And, again, that gives us our
inverse function, the function which maps the original 𝑦-values back onto their
original 𝑥-values.
Now, we’ve got to determine the
inverse of the function 𝑓 of 𝑥 is equal to the square root of 𝑥 plus three.
Well, there’re a couple of things
we need to think about here. The square root function is defined
as only being the positive square roots. And given that we can’t find the
square root of negative numbers, there are no real solutions. The 𝑥-values here must be greater
than or equal to zero. So the domain for this function
must be 𝑥 is greater than or equal to zero. Now, if 𝑓 of 𝑥 were just equal to
the square root of 𝑥, the graph would look something like that. But we’re adding three to all of
those values. So that moves the curve up to look
something like this. So our domain was that 𝑥 had to be
greater than or equal to zero. But that has the knock-on
implication that the 𝑦-values are gonna end up being greater than or equal to
three. So we’ll just need to bear that in
mind when we’re thinking about our answer.
So, again, we’re gonna start off by
just writing out the equation of the function. So 𝑦 is equal to the square root
of 𝑥 plus three. Then we’re gonna swap the 𝑥- and
𝑦-variables over. And then we’re gonna rearrange to
make 𝑦 the subject. So we can subtract three from both
sides and then square both sides. So that gives us our inverse
function of 𝑦 equals 𝑥 minus three all squared. Or 𝑓 inverse of 𝑥 is equal to 𝑥
minus three all squared. But, remember, that process of
rearranging and swapping the 𝑥- and 𝑦-variables was actually all about reflecting
in the line 𝑦 equals 𝑥. So this is the curve we were
looking for. But 𝑦 equals 𝑥 minus three all
squared actually carries on up here, like this. So we don’t really want this
bit.
But remember, the range of the
original function maps onto the domain of the inverse function. So if we set our new domain to be
𝑥 is greater than or equal to three, then we’re only looking at the bit of that
curve that we’re interested in. Now, remember, we swapped 𝑥 and 𝑦
when we were doing the calculation. So we swap 𝑥 and 𝑦 in the domain
and range as well. This will be our answer. The inverse function of 𝑥 is 𝑥
minus three all squared. And that only works for 𝑥 is
greater than or equal to three.
Okay then, one last example.
Determine the inverse of the
function 𝑓 of 𝑥 is equal to 𝑥 minus two all squared minus three where 𝑥 is
greater than or equal to two.
Now, before we attempt this
question, let’s try to sketch the function that we’re starting off with. So we’re gonna start off with it
looks like it’s some sort of variation on 𝑥 squared. So if we start off with the 𝑥
squared function, then this bit in these parentheses here is a translation of 𝑦
equals 𝑥 squared by positive two in the 𝑥-direction. And then we’re taking away three
from all of those results, the 𝑦-coordinates. So that’s a translation of negative
three in the 𝑦-direction. So doing this one first, we’ve got
𝑦 equals 𝑥 minus two all squared would look like that 𝑦 equals 𝑥 squared shifted
right two. And then shifting it down three is
gonna look something like, oops, something like that. So that minimum point there would
move down to negative three. But it would still have an
𝑥-coordinate of two.
But the question said that the
domain was only four 𝑥 is greater than or equal to two. So that’s this part of the curve
here going up. So we can rub out everything
else. So the resulting function is just
gonna look like that. Now let’s think about our domain
and range. Well, the question told us that the
𝑥-values we could use for inputs were greater than or equal to two, so our domain
is greater than or equal to two. And this is only generating answers
which are greater than or equal to negative three. So the range can only contain
answers of 𝑦 is greater than or equal to negative three. Now, having thought about that, we
can go on and try and solve the question.
We start off by writing 𝑦 is equal
to 𝑥 minus two squared minus three. Then we swap over the 𝑥- and
𝑦-variables and rearrange to make 𝑦 the subject. So, first, we can add three to both
sides. Then we can take the square roots
of both sides. And then, finally, add two to both
sides. And having swapped over our 𝑥- and
𝑦-variables, we now need to swap over the domain and the range. So now the domain is not 𝑥 is
greater than or equal to two. But it’s greater than or equal to
negative three. What was the range of the 𝑦-values
on the input function.
Now, if we think about the
reflection in 𝑦 equals 𝑥, then for our inverse function, obviously the 𝑥-values
that we can input are greater than or equal to negative three. And if we were asked for the range,
the output values we could get would be greater than or equal to two. So our final answer then is that
the inverse function 𝑓 inverse of 𝑥 is equal to the square root of 𝑥 plus three
or plus two, for 𝑥 is greater than or equal to negative three.
