### Video Transcript

In this video, weβre gonna take a
simple function π of π₯ equals π₯ squared and carry out some transformations. Weβre gonna translate the graph
either left or right or up or down. And weβre also gonna take a look at
the effect on the equation and the graph, side by side. Now hereβs the graph of the
function π of π₯ is equal to π₯ squared. Now, letβs think about some of the
points on that graph. π of zero is zero. If we input an π₯-value of zero
into that function, we get an output value, or π¦-coordinate, which is also
zero. And if we input a value of one, we
can see that the corresponding π¦-coordinate is also one. So with the function π of π₯
equals π₯ squared, weβre taking an input value and weβre squaring it and getting an
output value.

Now, it just so happens, the first
two numbers weβve tried, the input is the same as the output. But letβs try π of two. Well, π of two is two squared, and
two squared is four. So thatβs this point on the
graph. Then π of negative one is negative
one squared, which is one. So thatβs this point on the
graph. And π of negative two is negative
two squared, which is four. So thatβs this point on the
graph. And π of three and π of negative
three, both give output values of nine. So those two points from the graph
as well. Now weβve plotted π¦ equals π of
π₯, but what if we wanted to plot π¦ equals π of π₯ plus two. Now, remember, when plotting π¦
equals π of π₯, we took all of the π of π₯ values and used those as the
π¦-coordinates. So π¦ equals π of π₯ plus two,
weβre taking all of the π¦-coordinates and just adding two to them.

So when the π¦-coordinate was zero,
itβs going to become two in π of π₯ plus two. When the π¦-coordinate was one, we
add two to that to make three. When it was four, we add two to
that to make six, and so on for all the other points. So when weβre plotting π¦ equals π
of π₯ plus two, itβs gonna look exactly like π¦ equals π of π₯, but all the
π¦-coordinates have had two added to them. So weβre gonna get a curve that
looks like this. Weβve translated the function
positive two units in the π¦-direction. And that same effect wouldβve
occurred whatever the original function looked like. For example, if the graph π¦ equals
π of π₯ had looked like this, then for π¦ equals π of π₯ plus two, we just
increase all of those π¦-coordinates by two. And this is what the transformed
function looks like. And itβs important to notice that
itβs this plus two here whichβs been added to all the original π¦-coordinates, which
has transformed or translated that function in this case.

Now, letβs try adding negative two
to our function or subtracting two. Now when the π¦-coordinate was
zero, we subtract two to make it negative two. When it was one, we subtract two to
make it negative one. When it was four, we subtract two
to make it two, and so on for all the points on the curve. So the curve moves down two
places. It translates negative two in the
π¦-direction. So adding a constant to a function
translates its graph by that number of units in the π¦-direction. Now, letβs take a quick look at
what it does to the equation of the function.

If the equation of our function was
π of π₯ equals π₯ squared, then adding two to π of π₯ turns it into π₯ squared
plus two. All of these π¦-coordinates are the
same as these π¦-coordinates, but with this extra two added. And similarly, if we subtract two
from our function, all of these π¦-coordinates are the same as these π¦-coordinates
except they get two taken away. And if our function was something a
little bit more complicated like π of π₯ equals three π₯ squared minus two π₯ minus
seven and I added something, say five, then although the function can be simplified
to an expression like this, itβs still a matter of taking all the same
π¦-coordinates we had before but adding five to them. And likewise with subtracting, if I
took away seven from each side, the expression for the function may simplify to
this. But itβs still a matter of taking
these same π¦-coordinates but just subtracting seven from them.

So adding something to a function
translates the graph vertically upwards, and subtracting something from a function
translates the graph vertically downwards. And in both cases those
translations are parallel to the π¦-axis. Now letβs think about shifting left
and right. To do this, weβre adding or
subtracting numbers from the π₯-coordinate, but we do need to be a bit careful
because it plays out slightly differently to the up and down version. If π of π₯ is equal to π₯ squared,
then π of π₯ plus two is equal to π₯ plus two all squared, which is π₯ plus two
times π₯ plus two. And when I multiply those out and
simplify, I get π₯ squared plus four π₯ plus four. So by adding two to the
π₯-coordinate being input, I get a function which looks quite different to the
original function.

Letβs have a look at the graphs
then of π¦ equals π of π₯ and π¦ equals π of π₯ plus two. My dotted curve is π¦ equals π of
π₯ and remember thatβs π¦ equals π₯ squared. And our solid curve is π¦ equals π
of π₯ plus two. What we should remember is π¦
equals π₯ squared plus four π₯ plus four. Now you mightβve expected that this
bit here would be adding two to all of the π₯-coordinates on our curve, but thatβs
not whatβs happened. Itβs actually subtracted two from
all of the π₯-coordinates on our curve. This point here has moved to
here. This point here has moved to here,
and so on. Itβs shifted the graph to the left
two places, or translated it negative two in the π₯-direction.

Now letβs take a closer look and
try and understand why thatβs happened. So hereβs just the graph of π of
π₯ and remember π of π₯ is equal to π₯ squared. Now, letβs define a new function
called π of π₯ which is equal to π of π₯ plus two. Now to work out the π¦-coordinates
for the π of π₯ function, we can read them from the corresponding point on the π
of π₯ function. But the corresponding point is the
one thatβs got an π₯-coordinate which is two bigger. So for example, to work out the
π¦-coordinate when the π₯-coordinate is zero, in other words the value of π zero,
all we have to do is evaluate π of zero plus two, so π of two.

Now, π of two is this point here,
four. So π of zero is the same as π of
two. That four gets moved to here. And to work out the π¦-coordinate
for the π function that goes with an π₯-coordinate of one, we work out π of
one. But remember π of one is the same
as π of one plus two, so π of three. So to work out the π¦-coordinate
that goes with an π₯-coordinate of one on the π function, we need to find the
π¦-coordinate that goes with an π₯-coordinate of three on the π function; and that
is nine. So π of one is nine. This point here has effectively
moved two places to the left.

Letβs do one more π of negative
two. Well, π of negative two is equal
to π of negative two plus two, so thatβs π of zero. So the π¦-coordinate that goes with
an π₯-coordinate of negative two on the π function is the same as the π¦-coordinate
that goes with an π₯-coordinate of zero on the π function. And π of zero is zero. So on the π function, when π₯ is
equal to negative two, the π¦-coordinate will be zero. This point is mapping to this
point. So having done that to a few
different points, it soon becomes clear that the π of π₯ curve is being translated
two units to the left to get the π of π₯ curve. So adding two to the inputs of that
function has translated the graph two places to the left. And similarly, if we look at that
function π of π₯ minus two, when I expand that out and simplify, I get this
function here π₯ squared minus four π₯ plus four.

Now again, letβs look at the curves
of π of π₯ and π of π₯ minus two. Again the dotty curve is π¦ equals
π of π₯ and that was π₯ squared, remember. And the solid curve is π¦ equals π
of π₯ minus two which was π₯ squared minus four π₯ plus four. So subtracting two from the inputs
to our function translates the graph positive two units in the π₯-direction. So adding a constant to the inputs
translates the graph to the left, and subtracting a constant from the inputs
translates the graph to the right.

Now, letβs just summarise what
weβve learnt so far. If we see something like π of π₯
plus π, weβre adding π to all the π¦-coordinates so this translates π of π₯ by
positive π units in the π¦-direction. And that means if π is positive,
weβre translating upwards and if π is negative, weβre translating downwards. And if we encounter something like
π of π₯ plus π where weβre adding a constant to π₯ inside the brackets there in
the function, this translates π of π₯ by negative π units in the π₯-direction. And now if π is positive, weβre
translating the graph to the left and if π is negative, weβre translating the graph
to the right. Now, we can also combine these two
sorts of transformations.

For example, if π of π₯ equals
π₯ squared, describe the effect of the transformation π of π₯ minus five plus
three on the graph of π¦ equals π of π₯, and write and simplify the equation of
the transformed function.

So if π of π₯ is equal to π₯
squared, then π of π₯ minus five plus three is equal to π₯ minus five all
squared plus three. Now subtracting five from the
inputs is going to shift the whole curve to the right by five places. In other words, it translates
positive five units in the π₯-direction. And adding three to that result
is gonna add three to all the π¦-coordinates which is translating positive three
units in the π¦- direction. And itβs doing both of those
two things. So thatβs our answer for the
first part of the question. Now letβs go on and multiply
out and simplify the expression for our function. And that gives us our answer
for the second part of the question.

Now typical part b to that sort
of question might be to sketch π¦ equals π of π₯ minus five plus three on the
given diagram. So theyβve given us the graph
of the π¦ equals π of π₯ function and weβve got to translate it five places to
the right and three places upwards, as we worked out from the first part of the
question. So letβs just go through some
points on that graph and do the translations. Zero, zero goes five places to
the right and three places up here to five, three. One, one goes five to the right
and three up to six, four. And two, four goes five to the
right and three up to seven, seven, and so on with some more points.

Then now we just have to join
up all those translated points as accurately as we can, extending off up here
somewhere and extending off up here somewhere, and label it. Now it doesnβt have to be a
hundred percent accurate, but you do have to show the method and the key points
have to be accurately translated, in this case five places to the right and
three places up. So now letβs just look at a
couple of examples where they donβt give you the equation of the function; they
just give you the graph.

Given this graph of π¦ equals
π of π₯, sketch the graph of π¦ equals π of π₯ minus three.

Well, what do you think this is
doing? Weβre subtracting three from
the input π₯-coordinates before they go into the function. So the result of this, itβs
gonna be to translate that graph three places to the right. And my advice here would be to
pick some points which are gonna be easy to translate, to start off with. So this point here has got
integer coordinates, so just moving that three places to the right, is gonna
move it to here. This point here, three places
to the right is gonna move it to here. And this point here, three
places to the right is gonna move it to here.

Now, this point here is gonna
move three places to the right so one, two, three, thatβs about here. And this point here is gonna
move three places to the right so one, two, three, thatβs about here. And letβs pick a couple
more. This point up here is gonna
move three places to the right, so thatβs gonna move to here. And this point here is gonna
move three places to the right so one, two, three, thatβs gonna be about to
here. And one more, this point here
is gonna move three places so one, two, three, thatβs about here.

Now we just have to join them
up as accurately as we can, and so itβs going through in here so thatβs going up
and then down here. So again it doesnβt need to be
a hundred percent accurate, but we do need to show that weβve picked some
certain points and moved them exactly three places to the right.

And now this one.

Given this graph of π¦ equals
π of π₯, sketch the graph of π¦ equals π of π₯ plus three minus two.

First we need to think about
the transformation. This bit in here, what do you
think thatβs doing? Weβre adding three to all of
the input π₯-coordinates in the function. So thatβs gonna translate
negative three units in the π₯-direction. And how about this bit. Well weβre subtracting two from
the results, so weβre subtracting two from all of our π¦- coordinates. So that means weβre translating
negative two units in the π¦-direction. And weβre doing both of those
two things to our graph. So again weβre gonna take some
key points on the graph, do the transformations and then join up the dots. This point here is going
negative three in the π₯-direction and negative two in the π¦-direction, so
thatβs gonna end up here. This point here, negative three
in the π₯, negative two in the π¦, will end up here. Do the same for some more
points and then join them all up.

And just finally to check, make
sure that your curve is still the same shape and itβs been shifted left and down
by the appropriate amounts. Check these maximum and minimum
points here. So this maximum point here is
definitely going to the left three and down two and itβs mapped onto the maximum
point. This minimum point here has
gone left three, down two and that is still on a minimum point on my new version
of the curve.

So one final summary then, π of π₯
plus π means weβre adding π to the π¦-coordinate. And the π¦-coordinate is the output
from our function, so weβre adding π to the output. And this means if π is positive,
weβre translating the curve upwards and if π is negative, weβre translating the
curve downwards. And if we do π of π₯ plus π
inside the parentheses or the brackets there, weβre adding π to the π₯-coordinate
input. But beware, if you add to the
π₯-coordinate on the input, itβs gonna move the output to the left. If you subtract a value from the
input, itβs gonna move the graph to the right. And perhaps some slightly better
terminology, in the first case weβre translating positive π units in the
π¦-direction, and in the second case weβre translating negative π units in the
π₯-direction.