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Helpful graphic examples of different functions. Alg 2 level.
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Exploring Families of Functions Notes + Study Guide
General rules for transforming functions:
You can typically write functions as
a(bx – h) + k
Where “a”, “b”, “h”, and “k” are constants.
I. “a” represents the constant responsible for a vertical stretch or compression.
If |a| ≥1 (if “a” is not a fraction that’s less than 1) then the graph will be vertically
stretched.
If |a| ≤ 1 (if “a” is a fraction that’s less than 1 like 2
3 𝑜𝑟
7
8 for example) then the graph
will be vertically compressed.
In both cases, the y coordinates are multiplied by a, but when the graph is vertically
stretched it gets bigger in the y direction. When the graph is vertically compressed it
gets smaller in the y direction. You can tell this from the names “stretched” and
“compressed” too, but here are some sample graphs to show you what changing the
value of “a” can do:
In graph A, the red graph is the parent function y = (x-2) and the blue graph is the
modified function y = 2(x-2). So there is a vertical stretch of 2. You can see this
because for the same x coordinate, the y value of the blue line is twice that of the red
line. Take x = 10 for example. The y coordinate for the red line is 8, but the
GRAPH A GRAPH B
y coordinate for the blue line is 16 which = 2 x 8. So vertically stretching the graph
by 2 means all of the y coordinates get multiplied by 2.
In graph B, the red graph is still the parent function y = (x-2) but the blue graph is
the modified function y = 0.5(x-2). So there is a vertical compression by 0.5 since
|0.5| ≤ 1. Thus each y coordinate is multiplied by 0.5 and gets smaller. Take x = 10
as your sample point again. In the red graph, the y coordinate is 8 but in the blue
graph the y coordinate is 4 which equals 0.5 x 8.
If you get a question like this on a test and aren’t sure whether to shrink or
compress the function, you can always pick a value for x and solve for y like this:
So since the new y value is 2 x the old one, the function is stretched vertically.
If “a” is negative, then the graph is reflected over the x axis. This is because the y
coordinate changes to its opposite sign when it is multiplied by a negative. So if you
had a point (x, y), reflecting it over the x axis makes the new point (x, -y). We can see
this by graphing as well.
GRAPH C
In graph C, the red graph is the same
parent function y = (x-2) but the blue
graph is the modified function
y = -(x-2). “a” in this case is -1. So you
can see that each y coordinate in the red
graph is just the negative value for the
blue graph. Take a sample point again, x
= 10 for example. In the red graph, you
have the point (10, 8) so the y
coordinate = 8. In the blue graph
however, you have the point (10, -8) so
the y coordinate is -8. In other words,
each red y coordinate is simply
multiplied by “–a” (-1 in this case) to get
the blue y coordinate. So you’re still just
multiplying each y value by “a”, but since
“a” is negative here, you reflect each
point over the x axis.
FOR PARENT FUNCTION
y = (x - 2)
y = (10 – 2) y = 8
FOR MODIFIED FUNCTION
y = 2(x - 2)
y = 2(10 – 2)
y = 20 – 4 y = 16
GRAPH D GRAPH E
II. “b” represents the constant responsible for a horizontal stretch or compression. If |b| ≥1 (if “b” is not a fraction that’s less than 1) then the graph will be
horizontally compressed.
If |b| ≤ 1 (if “b” is a fraction that’s less than 1 like 2
3 𝑜𝑟
7
8 for example) then the graph
will be horizontally stretched.
This is more or less the opposite of what happens when |a| is greater than or less than 1. (If |a| ≥ 1, the graph is vertically stretched, but if |b| ≥ 1, the graph is
horizontally compressed.)
In both horizontal stretches and compressions, the new x coordinates are just the
old ones multiplied by the reciprocal of “b.” When the graph is horizontally
stretched it gets bigger in the x direction.
When the graph is horizontally compressed it gets smaller in the x direction.
Again, here are some graphs to show you what changing the value of “b” does”
In graph E, the red graph is the same parent function y = (x - 2) and the blue graph is
the modified function y = (2x – 2). So there is a horizontal compression by 2 in the
blue graph. This means that the x values in the blue graph are ½ the x values in the
red graph for the same y value. Thus the new x coordinates are the old ones
multiplied by the RECIPROCAL of 2, which is ½ . Take the sample point, y = 8. In the
red graph, if you find y = 8 and look at the x value, you can see the point is (10, 8) so
the x coordinate is 10. Do the same for the blue graph. At y = 8, x = 5, which equals
0.5 x 10 or ½ the x value from the red graph. So the graph shrinks horizontally. If
you get a question like this on a test and aren’t sure whether to shrink or compress
the function, you can always pick a value for y and solve for x like this.
FOR MODIFIED FUNCTION
y = (2x - 2)
8 = (2x – 2)
2x = 8 + 2
2x = 10
x = 5
x
FOR PARENT FUNCTION
y = (x - 2)
8 = (x – 2)
x = 8 + 2
x = 10
So you can tell that since the new x value is ½ the old one, the graph shrinks.
In graph B, the red graph is still the parent function y = (x-2) but the blue graph is
the modified function y = (0.5x-2). So there is a horizontal stretch since |0.5| ≤ 1.
Thus each x coordinate is multiplied by 2 (multiplied by the reciprocal of ½) and
gets bigger. Take y = 8 as your sample point again. In the red graph, the x coordinate
is 10 but in the blue graph the x coordinate is 20, which equals 2 x 10.
If “b” is negative, the graph is reflected over the y axis. This is because its x
coordinate changes to a negative when it is multiplied by its opposite sign, and in
order to do this, the points are reflected over the y axis. We can see this by graphing
as well.
In graph C, the red graph is the same
parent function y = (x-2) but the blue
graph is the modified function
y = (-x-2). “b” in this case is -1. So you
can see that each x coordinate in the red
graph is just the negative value for the
blue graph. Take a sample point again, y
= 2 for example. In the red graph, you
have the point (4, 2) so the y coordinate
= 4. In the blue graph however, you have
the point (-4, 2) so the x coordinate is -4.
In other words, each red x coordinate is
simply multiplied by “–b” (-1 in this
case) to get the blue x coordinate. So
you’re multiplying each x value by “b”,
but since “b” is negative here, you reflect each point over the y axis.
GRAPH F GRAPH G
FOR PARENT FUNCTION
y = (x)
First pick a y value on the graph… I
chose y = 0.
0 = x So the graph intercepts the y axis at x = 0
FOR MODIFIED FUNCTION
y = (x - 2)
Pick the same y value you chose for the
other one. In this case, it’s still y = 0
0 = (x – 2)
X = 2 So the graph intercepts the y axis at x = 2
III. “h” is the constant responsible for a horizontal phase shift (for moving the
function left or right).
(x – h) moves the parent function y = x “h” units right.
(x + h) moves the parent function y = x “h” units left.
While this might seem counterintuitive, we can prove this by graphing. Here are
some examples of how adding or subtracting a constant “h” changes a function:
In graph F, the red graph is the parent function y = x and the blue graph is the
modified function y = (x – 2). You can see that the blue graph is the same as the red
one except that is it moved 2 units to the right.
In graph G, the red graph is the parent function y = x and the blue graph is the
modified function y = (x + 2). You can see that the blue graph is the same as the red
one except that is it moved 2 units to the left.
If you get a question like this on a test and aren’t sure if the function moves to the
right or left, you can find a value of y that is in both graphs and try to find the x
values for them like this:
Since x = 2 is two units to the right from x = 0, the function moves 2 to the right
GRAPH H GRAPH I
IV. “k” is the constant responsible for a vertical phase shift (for moving the function
up or down).
x - k moves the parent function y = x “k” units down.
x + k moves the parent function y = x “k” units up.
We can prove this by graphing. Here are some examples of how adding or
subtracting a constant “k” changes a function:
In graph H, the red graph is the parent function y = (2x +1)2 and the blue graph is
the modified function y = (2x + 1)2 – 3. So you can see that the blue function is the
same as the red one but is shifted 3 units down.
In graph I, the red graph is the parent function y = (2x +1)2 and the blue graph is the
modified function y = (2x + 1)2 + 3. The blue function is the same as the red one but
is shifted 3 units up.
The key difference between the constants “h” and “k” is that “h” is applied only to x
whereas “k” is applied to the whole function (“h” is inside of the parentheses
containing x and “k” is outside.)
So the function (2x + 1)2 – 3 has a horizontal phase shift 1 unit to the left and a
vertical phase shift 3 units down.
GRAPH J GRAPH K
GRAPH L
V. Domain:
Domain is basically all of the x values that a function can be defined for (all of the x
values a function can have). Here are some examples of graphs and their domains.
Graph J is the function y = x. If we were to make the window of the graph huge,
(-10,000,000 to 10,000,000), for example, we would see that the graph still continues in both directions. Thus, since the x values can extend from −∞ to ∞, the domain of the
function is (−∞, ∞).
Graph K is the function y = x2. You can plug in any number for x so the domain is still (−∞, ∞).
In graph L, the function is
Y = 𝑥
𝑥2+3𝑥−4
We can factor the denominator into
(x +4)(x – 1) which means that x
cannot equal -4 or 1. Because of this,
there are vertical asymptotes at -4,
and 1 as shown by the dotted green
lines. Thus, there are no defined
values for x at -4 and 1.
The domain is
(−∞, −4) 𝑈 (−4, −1) 𝑈 (1, ∞)
GRAPH J GRAPH K
GRAPH L
VI. Range:
Range is basically all of the y values that a function can be defined for (all of the y
values a function can have). The same examples of graphs and their domains can be
used to find their ranges:
The range of Graph J is (-∞ , ∞) since there are negative values for y and positive
values.
The range of Graph K is (0, ∞) because there are no y values below 0 and there can’t
be any below 0. If you try plugging any negative x into the function y = x 2 you will
get a positive number for y. Thus, there cannot be any negative values and the range
begins at 0.
In graph L, the function is
Y = 𝑥
𝑥2+3𝑥−4
The range is (−∞, ∞)
Because there are y values that stretch in both directions.
VII. CHEAT SHEET:
For transformations of functions that follow the format a(bx – h) +k:
1. If |a| ≥ 1, function is vertically stretched
2. If |a| ≤ 1, function is vertically compressed
3. If |b| | ≥ 1, function is horizontally compressed
4. If |b| ≤ 1, function is horizontally stretched
5. If h is +
(x – (+h)) = (x – h)
function moves to the right
6. If h is –
(x – (-h)) = (x + h)
Function moves to the left
7. If k is + function moves up
8. If k is – function moves down
Domain and Range: 1. For even powered functions, domain = (−∞, ∞)
* Range = (0, ∞) UNLESS THERE IS A VERTICAL PHASE SHIFT
2. For odd powered functions, domain = (−∞, ∞)
Range = (−∞, ∞)
3. For absolute value functions, domain = (−∞, ∞)
* Range = (0, ∞) UNLESS THERE IS A VERTICAL PHASE SHIFT
* IF there is a vertical phase shift, the 0 will be replaced with the minimum y value the
function can have.