4. Transformations of Quadratic Functions
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Chapter 9
4.
Transformations of Quadratic Functions
This lesson delves into the fascinating world of quadratic functions, specifically focusing on how these functions can be transformed. It covers horizontal and vertical translations, stretches, and shrinks. For example, if you're an architect designing a parabolic arch, understanding these transformations can help you adjust the curve to fit specific dimensions. Similarly, in physics, these transformations can be used to model the trajectory of a projectile. The lesson also provides practical examples, like a student named Kriz who is tasked with various transformations of quadratic functions. It even discusses more complex transformations that combine multiple types of shifts and stretches.
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| | 14 Theory slides |
| | 10 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
Transformations of Quadratic Functions
Slide of 14
This lesson will discuss how to apply different transformations to quadratic functions.
Catch-Up and Review
Here are a few recommended readings before getting started with this lesson.
Challenge
Transforming the Graph of a Function
The graph of the quadratic function f(x)=-x2+2x+1 is drawn on the coordinate plane. By applying transformations to the parabola that corresponds to f, draw the graph of g(x)=-x2+4x.
Discussion
Reflection of Quadratic Functions
A reflection of a function is a transformation that flips a graph over a line called the line of reflection. A reflection in the x-axis is achieved by changing the sign of every output value of the function rule. In other words, the sign of the y-coordinate of every point on the graph of a function should be changed. Consider the quadratic parent function y=x2.
A reflection in the y-axis is instead achieved by changing the sign of every input value. However, since (-x)2 is equivalent to x2, reflecting y=x2 in the y-axis does not change the graph. For this reason, the reflection of the graph of another quadratic function will be shown.
Functiony=x2Reflection in the x-axisy=-x2
The reflection of the corresponding parabola can be shown on a coordinate plane.
Functiony=(x−1)2Reflection in the y-axisy=(-x−1)2
This transformation can also be shown on a coordinate plane. Example
Reflecting a Quadratic Function
Kriz is given an extra credit math assignment about quadratic functions and parabolas. The assignment consists of four tasks. For the first task, Kriz is given the graph of the function y=21(x+2)2+1.
Help Kriz with their extra credit assignment.
a Reflect the given parabola in the x-axis and write its corresponding equation.
b Reflect the given parabola in the y-axis and write its corresponding equation.
Answer
a Graph:
Equation: y=-21(x+2)2−1
b Graph:
Equation: y=21(x−2)2+1
Hint
a Recall that the graph of y=-f(x) is a reflection of the graph of y=f(x) in the x-axis.
b The graph of y=f(-x) is a reflection of the graph of y=f(x) in the y-axis.
Solution
a The reflection of the graph of y=f(x) in the x-axis is given by the equation of y=-f(x).
Given Functionf(x)=21(x+2)2+1Reflection in the x-axis-f(x)=-(21(x+2)2+1)⇕-f(x)=-21(x+2)2−1
Therefore, the graph of y=-21(x+2)2−1 is a reflection of the graph of y=21(x+2)2+1 in the x-axis.
b The reflection of the graph of y=f(x) in the y-axis is given by the equation of y=f(-x).
Given Functionf(x)=21(x+2)2+1Reflection in the y-axisf(-x)=21(-x+2)2+1
To obtained equation can be simplified.
Therefore, the graph of y=21(x−2)2+1 is a reflection of the graph of y=21(x+2)2+1 in the y-axis.
Explore
Stretching and Shrinking a Parabola
In the coordinate plane, the parabola that corresponds to the quadratic function y=af(bx) can be seen. Observe how the graph is vertically and horizontally stretched and shrunk by changing the values of a and b.
Discussion
Stretch and Shrink of Quadratic Functions
A function graph is vertically stretched or shrunk by multiplying the output of a function rule by some constant a, where a>0. This constant must be positive, otherwise a reflection is involved. Consider the quadratic function y=x2+1.
Similarly, a function graph is horizontally stretched or shrunk by multiplying the input of a function rule by some constant b, where b>0. Again, the constant must be positive because if it was negative, a reflection would be required.
The information about vertical and horizontal stretches and shrinks of the graph of a function f can be summed up in a table.
Functiony=x2+1Vertical Stretch/Shrinkby a Factor of ay=a(x2+1)
If a is greater than 1, the parabola is vertically stretched by a factor of a. Conversely, if a is less than 1, the graph is vertically shrunk by a factor of a. If a=1, then there is no stretch nor shrink. Here, the y-coordinates of all points on the graph are multiplied by the factor a.
Functiony=x2+1Horizontal Stretch/Shrinkby a Factor of by=(bx)2+1
In this case, if b is greater than 1, the graph is horizontally shrunk by a factor of b. Conversely, if b is less than 1, the graph is horizontally stretched by a factor of b. If b=1, then there is neither a stretch nor shrink of the graph. Here, the x-coordinates of all points on the graph are multiplied by the factor b1.
| Vertical | Horizontal | |
|---|---|---|
| Stretch | af(x), with a>1 | f(ax), with 0<a<1 |
| Shrink | af(x), with 0<a<1 | f(ax), with a>1 |
Example
Finding the Equation of a Stretch and a Shrink
Kriz's second task is about horizontal and vertical stretches and shrinks of parabolas.
They are given the quadratic function y=x2−3 and want to write the function rules of two related functions.
a A quadratic function whose graph is a vertical stretch of the graph of y=x2−3 by a factor of 2.
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b A quadratic function whose graph is a horizontal shrink of the graph of y=x2−3 by a factor of 4.
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Solution
a A function graph is vertically stretched or shrunk by multiplying the function rule by a positive constant. If the constant is greater than 1, the graph is vertically stretched. Therefore, if the graph of the given function is to be vertically stretched by a factor of 2, the function rule must be multiplied by 2.
Functiony=x2−3Vertical Stretchby a Factor of 2y=2(x2−3)
Recall that stretching a graph by a factor of 2 means multiplying the y-coordinates of all the points on the curve by a factor of 2. This can be seen in a coordinate plane.
y=2(x2−3) ⇔ y=2x2−6
b A function graph is horizontally stretched or shrunk by multiplying the input of the function rule by a positive constant. If the constant is greater than 1, the graph is horizontally shrunk. Therefore, to horizontally shrink the graph by a factor of 4, the input of the function rule must be multiplied by 4.
Functiony=x2−3Horizontal Shrinkby a Factor of 4y=(4x)2−3
This can be seen in a coordinate plane.
y=(4x)2−3 ⇔ y=16x2−3
Pop Quiz
Stating the Factor of a Stretch or a Shrink
The graph of the quadratic function y=x2 is shown in the coordinate plane. The graph of a horizontal or vertical stretch or shrink is also shown.
Example
Combining Transformations of Quadratic Functions
Kriz's assignment is getting more interesting, as the third task is about combining reflections with vertical and horizontal stretches and shrinks.
This time, they are given the quadratic function y=(x−1)2 and want to write the function rules of two other functions.
a A quadratic function whose graph is a vertical stretch by a factor of 3 followed by a reflection in the y-axis of the graph of y=(x−1)2.
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b A quadratic function whose graph is a horizontal shrink by a factor of 2 followed by a reflection in the x-axis of the graph of y=(x−1)2.
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Hint
Solution
a A function graph is vertically stretched or shrunk by multiplying the function rule by a positive constant. If the constant is greater than 1, the graph is vertically stretched. Therefore, if the graph of the given function is to be vertically stretched by a factor of 3, the function rule must be multiplied by 3.
Functiony=(x−1)2Vertical Stretchby a Factor of 3y=3(x−1)2
Furthermore, a function graph is reflected in the y-axis by changing the sign of the input.
Function y=3(x−1)2 Reflection in the y-axis y=3(-x−1)2
These transformations are illustrated by the following diagram.
y=3(-x−1)2
▼
Simplify right-hand side
NegBaseToPosPow
(-a)2=a2
y=3(-(-x−1))2
Distr
Distribute -1
y=3(x+1)2
ExpandPosPerfectSquare
(a+b)2=a2+2ab+b2
y=3(x2+2x(1)+12)
BaseOne
1a=1
y=3(x2+2x(1)+1)
IdPropMult
Identity Property of Multiplication
y=3(x2+2x+1)
Distr
Distribute 3
y=3x2+6x+3
b A function graph is horizontally stretched or shrunk by multiplying the input of the function rule by a positive constant. If the constant is greater than 1, the graph is horizontally shrunk. Therefore, to horizontally shrink the graph by a factor of 2, the input of the function rule must be multiplied by 2.
Functiony=(x−1)2Horizontal Shrinkby a Factor of 2y=(2x−1)2
Furthermore, a function graph is reflected in the x-axis by changing the sign of the output.
Function y=(2x−1)2 Reflection in the x-axis y=-(2x−1)2
The described transformations are demonstrated by the following diagram.
y=-(2x−1)2
▼
Simplify right-hand side
ExpandNegPerfectSquare
(a−b)2=a2−2ab+b2
y=-((2x)2−2(2x)(1)+12)
PowProdII
(ab)m=ambm
y=-(4x2−2(2x)(1)+12)
BaseOne
1a=1
y=-(4x2−2(2x)(1)+1)
Multiply
Multiply
y=-(4x2−4x+1)
Distr
Distribute -1
y=-4x2+4x−1
Explore
Translating a Graph
In the coordinate plane, the graph of the quadratic function y=(x−h)2+k can be seen. Observe how the graph is horizontally and vertically translated by changing the values of h and k.
Discussion
Translation of Quadratic Functions
A translation of a function is a transformation that shifts a graph vertically or horizontally. A vertical translation is achieved by adding some number to every output value of a function rule. Consider the quadratic function y=x2.
A horizontal translation is instead achieved by subtracting a number from every input value.
The vertical and horizontal translations of the graph of a function f can be summarized in a table.
Functiony=x2Vertical Translationby k Unitsy=x2+k
If k is a positive number, the translation is performed upwards. Conversely, if k is negative, the translation is performed downwards. If k=0, then there is no translation. This transformation can be shown on a coordinate plane.
Functiony=x2Horizontal Translationby h Unitsy=(x−h)2
In this case, if h is a positive number, the translation is performed to the right. Conversely, if h is negative, the translation is performed to the left. If h=0, then there is no translation. It is worth noting that since h is subtracted from x, if h is positive, then a number is subtracted from x. On the other hand, if h is negative, a number is added to the variable x.
| Translation | |
|---|---|
| Vertical | Horizontal |
| Upwards f(x)+k, with k>0 |
To the Right f(x−h), with h>0 |
| Downwards f(x)+k, with k<0 |
To the Left f(x−h), with h<0 |
Example
Translating a Quadratic Function
To finally finish the assignment and get the extra credit they need, Kriz has to finish the fourth task of the math assignment. This time, the graph of the quadratic parent function y=x2 is given.
By translating this quadratic function, Kriz wants to draw the graphs and write the equations of the following functions.
a A translation of the graph of y=x2 three units up.
b A translation of the graph of y=x2 two units to the right.
c A translation of the graph of y=x2 one unit down and three units to the left.
Answer
a Equation: y=x2+3
Graph:
b Equation: y=(x−2)2
Graph:
c Equation: y=(x+3)2−1
Graph:
Hint
b The graph of y=f(x−h) is a horizontal translation of the graph of y=f(x) by h units. If h is positive, the translation is to the right. If h is negative, the translation is to the left.
c The graph of y=f(x−h)+k is a horizontal translation followed by a vertical translation by h and k units, respectively.
Solution
a The graph of y=f(x)+k is a vertical translation of the graph of y=f(x) by k units. If k is positive, the translation is upwards. Conversely, if k is negative, then the translation is downwards.
Function y=x2 Translation 3 Units Upy=x2+3
This can be seen on the coordinate plane.
b The graph of y=f(x−h) is a horizontal translation of the graph of y=f(x) by h units. If h is positive, the translation is to the right. Conversely, if h is negative, then the translation is to the left. Note that since h is subtracted from x, if h is positive, then the number is subtracted from x. On the other hand, if h is negative, the number is added to x.
Function y=x2 Translation 2 Units to the Righty=(x−2)2
Here, since the given graph is to be translated 2 units to the right, the value of h is 2. Therefore, 2 is subtracted from the variable x.
This can be seen on the coordinate plane.
y=(x−2)2
▼
Simplify right-hand side
ExpandNegPerfectSquare
(a−b)2=a2−2ab+b2
y=x2−2x(2)+22
CommutativePropMult
Commutative Property of Multiplication
y=x2−2(2)x+22
CalcPow
Calculate power
y=x2−2(2)x+4
Multiply
Multiply
y=x2−4x+4
c The graph of y=f(x−h)+k is a horizontal translation by h units followed by a vertical translation by k. If h is positive, the horizontal translation is to the right, and if it is negative, this translation is to the left. Similarly, if k is positive, the vertical translation is upwards. If k is negative, this translation is downwards.
Function y=x2 Translation 1 Unit Down and 3 Units to the Lefty=(x+3)2−1
Again, special attention must be paid to the sign of h. This time, since the graph is to be translated 3 units to the left, the value of h is -3. Therefore, -3 must be subtracted from x. This is the same as adding 3 to x.
y=(x+3)2−1
▼
Simplify right-hand side
ExpandPosPerfectSquare
(a+b)2=a2+2ab+b2
y=x2+2x(3)+32−1
CommutativePropMult
Commutative Property of Multiplication
y=x2+2(3)x+32−1
CalcPow
Calculate power
y=x2+2(3)x+9−1
Multiply
Multiply
y=x2+6x+9−1
SubTerm
Subtract term
y=x2+6x+8
Pop Quiz
Stating the Translation
The graph of the quadratic function y=x2 and a vertical or horizontal translation are shown in the coordinate plane.
Closure
Transforming the Graph of a Function
With the topics learned in this lesson, the challenge presented at the beginning can be solved. The parabola that corresponds to the quadratic function f(x)=-x2+2x+1 is given.
Answer
Hint
Rewrite the function whose graph is to be drawn to clearly identify the transformations.
Solution
First, the function will be rewritten to identify the transformations.
The expression in parenthesis corresponds to the right-hand side of the equation of f(x) but with a negative sign for the variable x. Therefore, g(x)=3f(-x)+(-2). According to the topics learned in this lesson, the graph of g can be explained as a sequence of transformations. 
g(x)=-3x2−6x+1
▼
Rewrite
WriteDiff
Write as a difference
g(x)=-3x2−6x+3−2
FactorOut
Factor out 3
g(x)=3(-x2−2x+1)−2
NegBaseToPosBase
(-a)2=a2
g(x)=3(-(-x)2−2x+1)−2
AddNeg
a+(-b)=a−b
g(x)=3(-(-x)2+(-2x)+1)+(-2)
CommutativePropMult
Commutative Property of Multiplication
g(x)=3(-(-x)2+2(-x)+1)+(-2)
- Reflection in the y-axis.
- Vertical stretch by a factor of 3.
- Translation 2 units down.
f(x)=-2x2+12x−16
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A reflection of a function is a transformation that flips a graph over a line called the line of reflection. We can make a reflection in the y-axis by changing the sign of every input of the given function rule. This means that we need to find an expression for f( - x). g(x)=f( -x) ⇕ g(x)=- 2( -x)^2+12( -x)-16 Let's simplify the obtained equation!
We can verify the obtained result by graphing both quadratic functions on the same coordinate plane.
We can make a reflection in the x-axis by changing the sign of every output of the given function rule. This means that we need to find an expression for - f(x). h(x)= -f(x) ⇕ h(x)= - (- 2x^2+12x-16) Let's simplify the obtained equation by distributing - 1.
We can verify the obtained result by graphing both quadratic functions on the same coordinate plane.
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f(x)=-2x2+12x−16
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A function graph is vertically stretched by multiplying every output by a positive constant greater than 1. Therefore, if we want to vertically stretch the graph of the given function by a factor of 2, we multiply the function rule by 2. This means that we need to find an expression for 2f(x). g(x)=2f(x) ⇕ g(x)=2(- 2x^2+12x-16) We simplify the obtained equation by distributing 2.
We can verify the obtained result by graphing both quadratic functions on the same coordinate plane.
A function graph is vertically shrunk by multiplying every output by a positive constant less than 1. Therefore, if we want to vertically shrink the graph of the given function by a factor of 12, we multiply the function rule by 12. This means that we need to find an expression for 12f(x). h(x)=1/2f(x) ⇕ h(x)=1/2(- 2x^2+12x-16) We simplify the obtained equation by distributing 12.
We can verify the obtained result by graphing both quadratic functions on the same coordinate plane.
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f(x)=2x2−2
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A function graph is horizontally stretched by multiplying every input value by a positive constant less than 1. Therefore, if we want to horizontally stretch the graph of the given function by a factor of 12, we multiply the variable x by 12. This means that we need to find an expression for f( 12x). g(x)=f(1/2x) ⇕ g(x)=2(1/2x)^2-2 Let's simplify the obtained equation by applying properties of exponents.
We can verify the obtained result by graphing both quadratic functions on the same coordinate plane.
A function graph is horizontally shrunk by multiplying every input value by a positive constant greater than 1. Therefore, if we want to horizontally shrink the graph of the given function by a factor of 2, we multiply the variable x by 2. This means that we need to find an expression for f(2x). h(x)=f(2x) ⇕ h(x)=2(2x)^2-2 Let's simplify the obtained equation!
We can verify the obtained result by graphing both quadratic functions on the same coordinate plane.
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f(x)=-2x2+12x−16
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A translation of a function is a transformation that shifts a graph vertically or horizontally. We do a vertical translation by adding some number to every output value of a function rule. Therefore, if we want to translate the graph of f(x) 5 units up, we need to find an expression for f(x)+5. g(x)=f(x)+5 ⇕ g(x)=- 2x^2+12x-16+5 We can simplify the obtained equation by combining like terms.
We can verify the obtained result by graphing both quadratic functions on the same coordinate plane.
Previously, we said that a translation of a function is a transformation that shifts a graph vertically or horizontally. We perform a horizontal translation by subtracting some number to every input value of a function rule. Therefore, if we want to translate the graph of f(x) 4 units to the right, we need to find an expression for f(x-4). h(x)=f(x-4) ⇕ h(x)=- 2(x-4)^2+12(x-4)-16 Let's simplify the obtained equation!
We can verify the obtained result by graphing both quadratic functions on the same coordinate plane.
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Consider the following quadratic function.
h(x)=21(x−2)2
Describe how the graph of the given quadratic function is related to the graph of its parent function f(x)=x2.
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We want to describe how the graph of the given quadratic function is related to the graph of its parent function f(x)=x^2. To describe their relation, we will go through the process of transforming the parent function's graph of f(x)=x^2 into the graph of the given function. h(x)=1/2(x-2)^2 In order to do this, we need to consider two possible transformations.
- Vertical stretches and shrinks
- Horizontal translations
Let's consider them one at a time.
Vertical Stretch or Shrink
We have a vertical stretch when the function rule is multiplied by a positive number greater than one. If the output is multiplied by a positive number less than one, a vertical shrink takes place.
If the function rule is multiplied by a negative number, then a reflection takes place. In our case, the output of the function is multiplied by 12. Therefore, the graph of the parent function is vertically shrunk by a factor of 12.
Horizontal Translation
If an addition or subtraction is applied only to the x-variable, the graph is horizontally translated. In cases of addition, the graph is translated to the left. In cases of subtraction, it is moved to the right. In the given equation, 2 is being subtracted from x, so the previous graph is translated 2 units to the right.
Final Graph
Let's now graph the given function and the parent function f(x)=x^2 on the same coordinate plane.
Finally, let's summarize how the given function relates to the parent function f(x)=x^2.
- It is a vertical shrink by a factor of 12.
- It is a translation 2 units to the right.
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Consider the following quadratic function.
h(x)=-x2−2
Describe how the graph of the given quadratic function is related to the graph of its parent function f(x)=x2.
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We want to describe how the graph of the given quadratic function is related to the graph of its parent function f(x)=x^2. We will do this by transforming the graph of f(x)=x^2 into the graph of the given function. h(x)=- x^2-2 To make this transformation, we need to consider two possible types.
- Reflections
- Vertical translations
Let's consider them one at a time.
Reflections
Keep in mind that when a function rule is multiplied by - 1, the graph is reflected in the x-axis. Our given equation includes - x^2, indicating a reflection about the x-axis. Let's take a look at its graph.
Note how each x-coordinate stays the same, while on the other hand, each y-coordinate changes its sign.
Vertical Translations
If an addition or subtraction is applied to the whole function, the graph is vertically translated. In cases of addition, the graph is translated up. In cases of subtraction, it is moved downwards. In the given equation, 2 is subtracted from the whole function, which means that our graph is translated 2 units down.
Final Graph
Let's now graph the given function and the parent function f(x)=x^2 on the same coordinate plane.
Finally, let's summarize how the graph of the given function relates to the parent function f(x)=x^2.
- It is a reflection in the x-axis.
- It is a translation 2 units down.
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Transformations of Quadratic Functions
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