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### Course: Get ready for Precalculus > Unit 3

Lesson 4: Identifying function transformations# Identifying function transformations

Sal walks through several examples of how to write g(x) implicitly in terms of f(x) when g(x) is a shift or a reflection of f(x). Created by Sal Khan.

## Want to join the conversation?

- What is f(x) = |x| - 3

The fact that x is in between the absolute value sign confuses me. I know -3 would mean that we're going to the left on the horizontal plane, is that technically it?(20 votes)- f(x)=|x|-3. It's like f(x)=x-3 except the 3 is inside absolute value brackets. The only difference is that you will take the absolute value of the number you plug into x.

Remember that x just represents an unknown number.

To find f(x) (you can think of f(x) as being y), you need to plug a number into x.

f(x)=|x|-3

x=-2

Plug -2 into x

|-2|-3

The absolute value of any number is positive. Thus, -2 will become 2. Then subtract. 2-3=-1.

When x=-2 y=-1

(-2, -1)(37 votes)

- Are there more detailed videos that focus specifically on horizontal and vertical shifting and shrinking? Thanks(11 votes)
- I use this reference formula g(x)=a*f((1/b)x-h)+k

a is for vertical stretch/compression and reflecting across the x-axis.

b is for horizontal stretch/compression and reflecting across the y-axis. *It's 1/b because when a stretch or compression is in the brackets it uses the reciprocal aka one over that number.

h is the horizontal shift. *It's the opposite sign because it's in the brackets.

k is the vertical shift.(15 votes)

- At4:09, Why is it f(x-2) instead of f(x+2)? If you do minus 2, the values will get more negative, (from -3 to -5) but if you do plus two, then you would get the values of g...

Do you normally do the opposite when going left to right?(9 votes)- ayo did you figure it out? cause i am wondered too(3 votes)

- What would the transformation do if g(x)=(x+6)^2-10 and g(x) is in absolute value bars? Like this: |g(x)|.(6 votes)
- Taking the absolute value of a function reflects the negative parts over the x-axis, and leaves the positive parts unchanged. So a central segment of your parabola will be reflected so that it opens downward, with sharp corners at the roots.(4 votes)

- can some one help me?

What happens to the graph for f(x)=x when compared to the graph f(x)=x-5?(3 votes)- f(x)=x is equal to f(x)=x+0, just written in a more abstract way. This is useful when comparing to another linear functions such as your example.

f(x)=x-5 is simply just f(x)=x brought down by 5 units, hence the "-5" for the b term. I recommend using desmos for a more visual interpretation, as writing down explanations can be more convoluted.

Hopefully that helps !(8 votes)

- Could anyone ennumerate all the ways a function can be transformed? Thank you!(4 votes)
- Well, a function can be transformed the same way any geometric figure can:

They could be shifted/translated, reflected, rotated, dilated, or compressed. So that's pretty much all you can do with a function, in terms of transformations. Hope that answered your question!(5 votes)

- f(x)=x,g(x)=x+1

would the transformation of the problem be translation(4 votes)- Yep, for linear functions of the form mx+b m will stretch or shrink the function (Or rotate depending on how you look at it) and b translates. Then if m is negative you can look at it as being flipped over the x axis OR the y axis.

For all other functions, so powers, roots, logs, trig functions and everything else, here is what is hopefully an easy guide.

a*f(b(x+c))+d

so for example if f(x) is x^2 then the parts would be a(b(x+c))^2+d

a will stretch the graph by a factor of a vertically. so 5*f(x) would make a point (2,3) into (2,15) and (5,7) would become (5,35)

b will shrink the graph by a factor of 1/b horizontally, so for f(5x) a point (5,7) would become (1,3) and (10,11) would become (2,11)

c translates left if positive and right if negative so f(x-3) would make (4,6) into (7,6) and (6,9) into (9,9)

d translates up if positive and down if negative, so f(x)-8 would make the points (5,5) and (7,7) into (5,-3) and (7,-1)

Also should note -a flips the graph around the x axis and -b flips the graph around the y axis. Hope I didn't over explain, just proud of what I made tbh(4 votes)

- I like how everyone is asking about certain math questions and the typical "where in real life would this be useful" kinda thing, and yet I seem to be the only one who's wondering about that fifth graph down at the bottom. What's that doing down there? Why did Sal not do any problems on that one but still did problems on the other four? These are legitimate questions.(4 votes)
- Maybe he had been planning to use it, but then he ran over time or something. Or it could be for another video.(2 votes)

- I have a really hard time understanding if a graph is being compressed vertically or if it is being stretched horizontally and vice versa. You could make a case for both and the help feature on the exercise "Identify function transformations" does not explain why they came to that conclusion.(4 votes)
- you can tell because a horizontal stretch wouldnt really change the y values on the graph but rather the locations that the y value is achieved for example, say there was a point (1,4) if this was stretched horizontally, the x value would change for example to (4,4) however you would notice that y=4 is still attained on the graph at some point. While a vertical stretch would only impact the y values, while the x values would remain the same.(1 vote)

- Okay this makes sense, but in 5 or so minutes, I am bound to forget everything he just said. Its confusing. I need an easier way to remember this.(4 votes)
- Just take some time to review and think about what he discussed and work on gaining an intuitive understanding of it.(1 vote)

## Video transcript

So this red curve is
the graph of f of x. And this blue curve is
the graph of g of x. And I want to try to express
g of x in terms of f of x. And so let's see
how they're related. So we pick any x. And we could start right
here at the vertex of f of x. And we see that, at least
at that point, g of x is exactly 1 higher than that. So g of 2-- I could
write this down-- g of 2 is equal to f of 2 plus 1. Let's see if that's
true for any x. So then we can just
sample over here. Let's see, f of 4
is right over here. g of 4 is one more than that. f of 6 is right here. g of 6 is 1 more than that. So it looks like if we pick
any point over here-- even though there's a little bit
of an optical illusion-- it looks like they
get closer together. They do if you look
try to find the closest distance between the two. But if you look at
vertical distance you see that it
stays a constant 1. So we can actually
generalize this. This is true for
any x. g of x is equal to f of x is
equal to f of x plus 1. Let's do a few more
examples of this. So right over here, here
is f of x in red again, and here is g of x. And so let's say we picked
x equals negative 4. This is f of negative 4. And we see g of negative
4 is 2 less than that. And we see whatever f of
x is, g of x-- no matter what x we pick-- g of x
seems to be exactly 2 less. g of x is exactly 2 less. So in this case, very
similar to the other one, g of x is going to
be equal to f of x. But instead of
adding, we're going to subtract 2 from f
of x. f of x minus 2. Let's do a few more examples. So here we have f
of x in red again. I'll label it. f of x. And here is g of x. So let's think about
it a little bit. Let's pick an
arbitrary point here. Let's say we have in red here,
this point right over there is the value of f of negative 3. This is negative 3. This is the point
negative 3, f of 3. Now g hits that same value
when x is equal to negative 1. So let's think about this. g of negative 1 is equal
to f of negative 3. And we could do that
with a bunch of points. We could see that g of 0, which
is right there-- let me do it in a color you can
see-- g of 0 is equivalent to f of negative 2. So let me write that down. g of 0 is equal to
f of negative 2. We could keep doing that. We could say g of 1,
which is right over here. This is 1. g of 1 is equal to
f of negative 1. g of 1 is equal to
f of negative 1. So I think you see
the pattern here. g of whatever is equal to the
function evaluated at 2 less than whatever is here. So we could say that g of
x is equal to f of-- well it's going to be 2 less than x. So f of x minus 2. So this is the relationship. g of x is equal
to f of x minus 2. And it's important
to realize here. When I get f of x minus 2 here--
and remember the function is being evaluated, this is the
input. x minus 2 is the input. When I subtract the 2, this
is shifting the function to the right, which is a
little bit counter-intuitive unless you go through this
exercise right over here. So g of x is equal
to f of x minus 2. If it was f of x plus 2 we
would have actually shifted f to the left. Now let's think about this one. This one seems kind of wacky. So first of all,
g of x, it almost looks like a mirror
image but it looks like it's been flattened out. So let's think of it this way. Let's take the mirror
image of what g of x is. So I'm going to try my best to
take the mirror image of it. So let's see... It gets to about
2 there, then it gets pretty close to
1 right over there. And then it gets about
right over there. So if I were to take
its mirror image, it looks something like this. Its mirror image if I were to
reflect it across the x-axis. It looks something like this. So this right over
here we would call-- so if this is g of x,
when we flip it that way, this is the negative g of x. When x equals 4, g of
x looks like it's about negative 3 and 1/2. You take the negative of
that, you get positive. I guess it should
be closer to here-- You get positive
3 and 1/2 if you were to take the
exact mirror image. So that's negative g of x. But that still doesn't get us. It looks like we
actually have to triple this value for any point. And you see it here. This gets to 2, but
we need to get to 6. This gets to 1, but
we need to get to 3. So it looks like this
red graph right over here is 3 times this graph. So this is 3 times
negative g of x, which is equal to
negative 3 g of x. So here we have f of x is equal
to negative 3 times g of x. And if we wanted to solve for
g of x, right-- g of x in terms of f of x-- we would
write, dividing both sides by negative 3, g of x is
equal to negative 1/3 f of x.