2. Functions and Function Notation
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Chapter 3
2.
Functions and Function Notation
Functions and their notations play a pivotal role in algebra. This lesson delves into the nuances of how functions can be combined, not just through basic operations like addition or subtraction, but also through compositions. Compositions of functions involve evaluating one function using another, offering a layered approach to understanding mathematical relationships. Function notation, on the other hand, provides a standardized way to represent these relationships. It is essential to grasp these concepts as they form the foundation for more advanced mathematical studies. By mastering function notation and understanding compositions, one can tackle complex problems with ease and confidence.
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| | 10 Theory slides |
| | 9 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
Functions and Function Notation
Slide of 10
Once a relation is known to be a function, it is handy to write it in a way that directly gives essential information such as the independent and dependent variables. This is where function notation comes to the rescue. This lesson will explore some applications of function notation and how this way of representing functions helps combine different functions.
Catch-Up and Review
Here are a few recommended readings before getting started with this lesson.
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Evaluating Functions at Another Function
As with algebraic expressions, functions can also be evaluated at a specific input. In the following applet, drag a function into each empty box to explore the outputs obtained when a function is evaluated at another function.
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Discussion
Representing Functions Using Function Notation
There are different ways to represent a function — using tables of values, mapping diagrams, graphs, and equations. However, aside from these, one of the most common ways is using function notation.
Concept
Function Notation
Function notation is a special way to write functions that explicitly shows that y is a function of x — in other words, that y depends on x. Function notation is symbolically expressed as y=f(x) and read y equals f of x.
Equations that are functions can be written using function notation.
ccc
Equation & & Function Notation [1ex]
y=-5x+4 & & f(x) = -5x+4
Notice that y has been replaced by f(x). In function notation, x represents an element of the domain and f(x) represents the element of the range that corresponds to x. When written in function notation, the expression that describes how to convert an input into an output — the right-hand side expression — is called the function rule.
Besides f, other letters such as g or h can be used to name the function. Similarly, letters other than x can name the independent variable.
Extra
Interpreting Function NotationTo interpret an equation given in function notation, it is necessary to understand what both sides of f(x)=k mean. For example, consider the following equation.
f( 3) = 12
Here, f( 3) denotes that the function's input is x= 3 and that 12 is the output corresponding to this input.
f( 3) = 12 ⇓ The output offwhenx= 3 is 12.
Now, consider a different scenario. Let w(t)=200t be a function that describes the number of words Kevin reads in t minutes. The following statements are true for this function.
w(4) and w(t)=900
Here, w(4) is the number of words that Kevin reads in 4 minutes and can be found by evaluating the function for t=4. However, the input is not a particular number in the second statement. In such cases, the statement can be interpreted as a question.
w(t)=900 ⇓ For which value oft is the output equal to900?
Based on the context, the second statement asks how many minutes it takes Kevin to read 900 words. To find this value of t, the equation w(t)=900 has to be solved for t.
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Discussion
Evaluating Functions and Finding a Certain Input
In the same way that an expression can be evaluated at a particular x-value, functions can also be evaluated at a specific input. Furthermore, it is also possible to determine the input that produces a specific output.
Method
Evaluating a Function
Evaluating a function involves determining the value of the function when its independent variable is set to a specific value. This is done by substituting the given input value for the variable and evaluating the function rule. As an example, consider the value of the following function when x=4. f(x)=3x+4 To evaluate a function for a particular input, there are two steps to follow.
1
Substitute the Input for x
expand_more Substitute the given input value for every instance of the independent variable in the function. In this case, 4 is substituted for x.
2
Evaluate the Expression
expand_more Next, evaluate the resulting expression by performing the required operations. The simplified value will be the output for the given input value.
As shown, when the input is 4, the output of the function is 16.
Method
Finding the Input of a Function
Given a function, it is possible to find the input that produces a certain output. This is done by substituting the given output value for the dependent variable and then solving for the independent variable. For the following function, try finding the x-value for which f(x)=21. f(x)=4x-3 To find the input that produces a certain output, there are two steps to follow.
1
Substitute the Output for f(x)
expand_more Start by substituting the given output value for f(x). In this case, it means substituting 21 for f(x).
2
Solve for x
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Example
Modeling With Functions
For the last few days of the long holiday, Izabella and Emily visited a city they did not know. Before returning home, they went into a souvenir shop where all the souvenirs were priced the same. After picking out some gifts and just before paying, Emily found a $3 gift card in her purse.
The function C(s)=4.5s-3 represents the cost, in dollars, of buying s souvenirs with a $3 gift card.
a
How much will the girls pay if they buy ten souvenirs?
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b
How many souvenirs can the girls buy with $69?
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Hint
b
Find the value of s for which the output is 69. Substitute 69 for C(s) and solve the resulting equation for s.
Solution
a
Based on the given information, finding how much will the girls pay for buying ten souvenirs is equivalent to finding C(10). This can be done by substituting 10 for s into the function rule and simplifying the right-hand side.
C(s)=4.5s-3
Substitute
s= 10
C( 10) = 4.5( 10)-3
Multiply
Multiply
C(10) = 45-3
SubTerm
Subtract term
C(10) = 42
The statement C(10)=42 shows that when the input is s=10, the output is 42. In this context, it means that the girls will pay $42 for ten souvenirs.
b
In this context, C(s) gives the cost of buying s souvenirs. Therefore, asking how many souvenirs the girls can buy with $69 is the same as asking for which value of s the output of C is equal to 69.
C(s)=69 To find such value of s, in the function rule, substitute 69 for C(s) and solve the resulting equation for s.
C(s)=4.5s-3
Substitute
C(s)= 69
69 = 4.5s-3
▼
Solve for s
AddEqn
LHS+3=RHS+3
72 = 4.5s
DivEqn
.LHS /4.5.=.RHS /4.5.
72/4.5 = s
CalcQuot
Calculate quotient
16 = s
RearrangeEqn
Rearrange equation
s=16
As shown, the output is 69 when s=16. In other words, the girls can buy 16 souvenirs with $69.
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Example
Evaluating Functions
While walking downtown, Emily and Izabella walked into a craft store and saw a beautiful wooden jewelry box. The craftsman said that the price of the box is half its volume, which is given by the function V(x)=9(x+1)-3(3-x), where x is the length of an inner side of the box — a measure that the girls have to provide.
a
Emily wants to buy a box where the length of the inner side of the box is 1 inch. How much will she pay for this box?
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b
Izabella paid $10.50 for her box. What is the length of the inner side of the box she asked for?
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Hint
a
Start by finding the volume of the box for the measure Emily chose. To do this, find V(1). The price is half the volume.
b
Use the given price to find the volume of Izabella's box. Then, substitute this volume for V(x) in the function rule and solve the resulting equation for x.
Solution
a
Since the price depends on the volume, the volume of the box needs to be calculated first. Note that the volume of a box where the inner side is 1 inch long is given by V(1). Substitute 1 for x into the function rule and simplify the right-hand side.
V(x) = 9(x+1)-3(3-x)
▼
Substitute 1 for x and simplify
Substitute
x= 1
V( 1) = 9( 1+1)-3(3- 1)
AddSubTerms
Add and subtract terms
V(1) = 9(2)-3(2)
Multiply
Multiply
V(1) = 18-6
SubTerm
Subtract term
V(1) = 12
Therefore, the volume of the box Emily wants is 12 cubic inches. Since the price is half the volume, Emily will pay $6 for such a box.
b
To find the length of the inner side of Izabella's box, the volume of the box needs to be calculated first. If the price is half the volume of the box, then the volume of the box is twice the price paid. Therefore, the volume of the box will be found using the fact that Izabella paid $ 10.50 for the box.
2* 10.50 = 21 in^3 The volume of Izabella's box is 21 cubic inches. The next step is to find the value of x that produced this volume. To do so, substitute V(x) for 21 in the function rule and solve the equation for x.
V(x)= 9(x+1) - 3(3-x)
Substitute
V(x)= 21
21 = 9(x+1) - 3(3-x)
▼
Solve for x
Distr
Distribute 9 and -3
21 = 9x + 9 - 9 + 3x
AddSubTerms
Add and subtract terms
21 = 12x
DivEqn
.LHS /12.=.RHS /12.
21/12 = x
CalcQuot
Calculate quotient
1.75 = x
RearrangeEqn
Rearrange equation
x=1.75
Consequently, the output is 21 when x=1.75. Therefore, the length of the inner side of the box Izabella asked for is 1.75 inches.
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Pop Quiz
Evaluating Different Functions
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Discussion
Combining Functions
Two functions f(x) and g(x) can be combined into a sum, a difference, a product, or a quotient through the following formulas. (f+g)(x) &= f(x)+g(x) (f-g)(x) &= f(x)-g(x) (f* g)(x) &= f(x)* g(x) [0.1cm] (f/g) (x) &= f(x)/g(x), g(x)≠ 0 However, apart from these four operations, two functions can also be combined into a composition.
Concept
Composite Function
A composite function, or a composition of functions, combines two or more functions, which produces a new function. In a composition, the outputs produced by one function are the inputs of the other function. The composition of the functions f and g is denoted as f(g(x)) or (f∘ g)(x).
f(x)=2x+1
▼
Substitute g(x) for x and evaluate
Substitute
x= g(x)
f( g(x)) = 2 g(x) + 1
Substitute
f(g(x))= h(x)
h(x) = 2g(x) + 1
Substitute
g(x)= x-5
h(x) = 2( x-5) + 1
Distr
Distribute 2
h(x) = 2x - 10 + 1
AddTerms
Add terms
h(x) = 2x - 9
Extra
Parts of a CompositionExplore the parts of a composition by moving the magnifying glass.
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Example
Applying Discounts Through Compositions of Functions
Izabella and Emily are shopping for pants. Both girls have a $3 discount coupon for the store. After choosing a pair of pants each, they discovered that the pants are 25 % off the marked price. The girls could not agree which order of discount would save them more money, so the cashier allowed them to apply the discounts in whichever order they wanted.
The function f(x)=x-3 represents the effect of applying only the coupon to the marked price, while g(x)= 34x represents the effect of applying only the store's sale discount to the marked price. In both functions, x represents the marked price.
a
Write a function I(x) that represents how much Izabella will pay if she uses the coupon first and then applies the store's sale discount.
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b
Write a function E(x) that represents how much Emily will pay if she uses the store's discount first and then applies the coupon.
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c
Which of them saved more money?
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Hint
a
Applying the coupon first means to apply f to the marked price. Then, g must be applied to the resulting output. Therefore, I(x) is equal to g(f(x)). To find how much Izabella paid, evaluate I(31).
b
This time, apply the store's discount first — that is, start by applying g to the marked price. Then, apply f to the resulting output. In other words, E(x) equals f(g(x)). To determine the marked price, equate E(x) to the amount paid by Izabella. Then, solve the resulting equation for x.
c
Compare the total amount paid by each girl and the marked price of the chosen pair of pants.
Solution
a
Since Izabella decided to apply the coupon first, the function f will be applied first to the marked price.
The store's sale discount will be applied to the resulting output. This means that g will be applied to the previous output.
Therefore, the composition of g and f represents the cost that Izabella will pay, based on her decision. This means that I(x)=g(f(x)). To find this composition, in g(x)= 34x, substitute f(x) for x.
g(x)=3/4x
Substitute
x= f(x)
g( f(x))=3/4 f(x)
Substitute
g(f(x))= I(x)
I(x)=3/4f(x)
Substitute
f(x)= x-3
I(x)=3/4( x-3)
Izabella chose a pair of pants with a marked price of $31. To find how much she paid for them, evaluate I(x) at x= 31.
I(x)=3/4(x-3)
▼
Substitute 31 for x and evaluate
Substitute
x= 31
I( 31)=3/4( 31-3)
SubTerm
Subtract term
I(31)=3/4(28)
MoveRightFacToNum
a/c* b = a* b/c
I(31) = 84/4
CalcQuot
Calculate quotient
I(31) = 21
Consequently, Izabella paid $21 for the pair of pants she chose.
b
Emily decided to apply the discounts in the opposite order — that is, she chose to apply the store's sale discount first. For her purchase, the function g will be applied to the marked price.
Next, the coupon will be applied. Therefore, the function f will be applied to the previous output.
Consequently, the composition of f and g represents the cost that Emily will pay, based on the order she of discounts she chose — E(x)=f(g(x)). To calculate this composition, evaluate f(x) at g(x).
f(x)=x-3
Substitute
x= g(x)
f( g(x)) = g(x)-3
Substitute
f(g(x))= E(x)
E(x)=g(x)-3
Substitute
g(x)= 3/4x
E(x)= 3/4x-3
Emily paid the same amount as Izabella, which means that Emily paid $21. To determine the original cost of the pair of pants that Emily chose, substitute 21 for E(x) and solve the resulting equation for x.
E(x)= 3/4x-3
Substitute
E(x)= 21
21 = 3/4x-3
▼
Solve for x
AddEqn
LHS+3=RHS+3
24 = 3/4x
MultEqn
LHS * 4=RHS* 4
96 = 3x
DivEqn
.LHS /3.=.RHS /3.
32 = x
RearrangeEqn
Rearrange equation
x = 32
In conclusion, the marked price of the pants Emily chose was $32.
c
To determine who saved more money, it would help to organize the results obtained in a table.
| Marked Price | Amount Paid | |
|---|---|---|
| Izabella's Pants | $31 | $21 |
| Emily's Pants | $32 | $21 |
As shown, both girls paid the same amount of money, but the pair of pants that Emily chose was more expensive. As such, Emily saved more money from the discounts. In fact, if Izabella had applied the discounts as Emily did, she could have paid $20.25 for the same pair of pants. E(31) = 3/4(31)-3 = 20.25 In contrast, if Emily had applied the discounts as Izabella did, she would have paid $21.75. Regardless of who applied the discounts better, both girls got a big discount on their purchase and went home happy.
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Closure
A Special Case of Compositions of Functions
As shown, the composition of functions is not commutative, implying that, in general, f(g(x))≠ g(f(x)). However, for those functions that do commute, there is a special case. Consider, for example, the following pair of functions. f(x) = 3x - 6 and g(x) = 1/3x + 2 For these two functions, start by finding f(g(x)).
f(x) = 3x - 6
▼
Substitute g(x) for x and simplify
f( 1/3x + 2) = 3( 1/3x + 2) - 6
f(1/3x + 2) = x
g(x) = 1/3x + 2
▼
Substitute f(x) for x and simplify
g( 3x-6) = 1/3( 3x-6) + 2
▼
Simplify right-hand side
Distr
Distribute 1/3
g(3x-6) = 1/3* 3x-1/3* 6 + 2
a * 1/a=1
g(3x-6) = 1* x - 1/3* 6 + 2
MultByOne
a * 1=a
g(3x-6) = x - 1/3* 6 + 2
MoveRightFacToNum
a/c* b = a* b/c
g(3x-6) = x - 6/3 + 2
CalcQuot
Calculate quotient
g(3x-6) = x - 2 + 2
AddTerms
Add terms
g(3x-6) = x
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Functions and Function Notation
Exercise 2.1
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We are told that the hourly rate in the parking lot is $2.50 and that a flat fee is also charged. Price per hour & → $2.50 Flat fee & → $0.75 Based on this information, the cost for parking a car equals the product of the hourly rate and the number of hours parked plus the flat fee. Let's translate this phrase into an algebraic expression. C(t) = 2.5 t + 0.75 This function represents the cost of parking a car for t hours.
Let's start by writing the function we found that models the cost of parking a car for t hours. C(t) = 2.5t + 0.75 We know Kevin parked his car for four and a half hours. To determine how much will he pay when he leaves, we will evaluate C(t) at t=4.5.
Kevin will pay $12 for parking the car for four and a half hours.
The fact that Jordan paid $14.50 means that the function C(t) is equal to 14.50. C(t) = 14.50 We want to find the value of t that produced 14.50 as the output. To find it, we will substitute 14.50 for C(t) into the function rule found in the first part and solve the resulting equation for t.
We found that the output of C(t) is 14.50 when the input is 5.5. This means that Jordan parked her car for 5.5 hours.
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Solution
paybutton. It turns out that this pair of pink and blue Steezys are 15 % off the displayed price.
The function G(x)=x-10 represents the effect of applying only the gift card to the displayed price and S(x)= 1720x represents the effect of applying only the store's discount to the displayed price. In both functions, x represents the shown price.
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Applying the gift card first is the same as applying G to the displayed price, so we will start by applying G to x. x → G(x) Next, Dylan will apply the store's discount. This means that he will apply the function S to the output of G. x → G(x) → S( G(x) ) Therefore, the composition of S of G represents the cost that Dylan will pay, based on his decision. That is, f(x)=S(G(x)). To find this composition, we will substitute G(x) for x in the function rule of S(x).
Finally, to find the explicit rule of f(x), we will substitute x-10 for G(x).
In contrast to the previous part, now Dylan wants to apply the discounts in the opposite order. That is, he wants to apply the store's discount first. Therefore, the function S will be applied to the displayed price. x → S(x) Then Dylan will apply the gift card. This means that he will apply the function G to the output of S. x → S(x) → G(S(x)) The composition of G of S represents the cost that Dylan will pay, based on the order he picked. That is, g(x)=G(S(x)). As before, to find this composition, we will start by writing the function rule of G(x) and then substitute S(x) for x.
To determine the order that gives the maximum benefit to Dylan, we will find f(24) and g(24) and compare them.
This means that by applying the gift card first and the store's discount second, Dylan will pay $11.90 for the shoes. Let's now find g(24).
From this, we can say that by applying the store's discount first and the gift card second, Dylan will pay $10.40 for the shoes, which is $1.50 less than when using the opposite discount order. As such, the answer is option B. Store's discount first, then gift card ✓
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Solution
Consider the function f(x)=ax + b, where a and b are positive real numbers. The function f(x) satisfies the following equation. f(f(x)) = 25x + 42 Determine the explicit function rule that defines f(x).
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To determine the function rule that defines f(x), we need to find the values of a and b involved in the general form of f(x). f(x) = ax + b We can find these values by using the given equation. f(f(x) ) = 25x+42 The left-hand side of the equation implies that we need to perform the composition of f(x) with itself. To do this, in the general function rule, we substitute f(x) for x.
We can now substitute ax+b for f(x) on the right-hand side.
The resulting expression on the right-hand side should be equal to 25x + 42. For them to be equal, the coefficients of the x-terms and the constant term must be the same on both sides. Let's highlight them. a^2x + ab+b = 25x+42 From this equation, we see that a^2 must equal 25 and that ab+b must equal 42. Therefore, there are two possible values for a, 5 and -5. 5^2 &= 25 [1ex] ( -5)^2 &= 25 However, we are told that a is positive, which leads us to discard the negative option. Therefore, a=5. With this information, let's find b.
Since we already know the values of a and b, we can write the function rule that defines f(x). f(x) = ax+b ⇒ f(x) = 5x + 7
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Functions and Function Notation
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