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**4-3 Writing Functions Warm Up Lesson Presentation Lesson Quiz**

Holt Algebra 1 Warm Up Lesson Presentation Lesson Quiz

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**Warm Up Evaluate each expression for a = 2, b = –3, and c = 8.**

2. ab – c 3. 26 –14 1 2 c + b 1 4. 4c – b 35 5. ba + c 17

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**Objectives Identify independent and dependent variables.**

Write an equation in function notation and evaluate a function for given input values.

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**Vocabulary independent variable dependent variable function rule**

function notation

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**Example 1: Using a Table to Write an Equation**

Determine a relationship between the x- and y-values. Write an equation. x y 5 10 15 20 1 2 3 4 Step 1 List possible relationships between the first x and y-values. 5 – 4 = 1 and

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Example 1 Continued Step 2 Determine which relationship works for the other x- and y- values. 10 – 4 and 15 – 4 and 20 – 4 and The value of y is one-fifth, , of x. Step 3 Write an equation. or The value of y is one-fifth of x.

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Check It Out! Example 1 Determine a relationship between the x- and y-values. Write an equation. {(1, 3), (2, 6), (3, 9), (4, 12)} x 1 2 3 4 y 3 6 9 12 Step 1 List possible relationships between the first x- and y-values. 1 3 = 3 and = 3

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**Check It Out! Example 1 Continued**

Step 2 Determine which relationship works for the other x- and y- values. 2 • 3 = 6 3 • 3 = 9 4 • 3 = 12 2 + 2 6 3 + 2 9 4 + 2 12 The value of y is 3 times x. Step 3 Write an equation. y = 3x The value of y is 3 times x.

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The equation in Example 1 describes a function because for each x-value (input), there is only one y-value (output).

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**The input of a function is the independent variable**

The input of a function is the independent variable. The output of a function is the dependent variable. The value of the dependent variable depends on, or is a function of, the value of the independent variable.

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**Example 2A: Identifying Independent and Dependent Variables**

Identify the independent and dependent variables in the situation. A painter must measure a room before deciding how much paint to buy. The amount of paint depends on the measurement of a room. Dependent: amount of paint Independent: measurement of the room

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**Example 2B: Identifying Independent and Dependent Variables**

Identify the independent and dependent variables in the situation. The height of a candle decrease d centimeters for every hour it burns. The height of a candle depends on the number of hours it burns. Dependent: height of candle Independent: time

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**Example 2C: Identifying Independent and Dependent Variables**

Identify the independent and dependent variables in the situation. A veterinarian must weight an animal before determining the amount of medication. The amount of medication depends on the weight of an animal. Dependent: amount of medication Independent: weight of animal

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Helpful Hint There are several different ways to describe the variables of a function. Independent Variable Dependent Variable x-values y-values Domain Range Input Output x f(x)

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Check It Out! Example 2a Identify the independent and dependent variable in the situation. A company charges $10 per hour to rent a jackhammer. The cost to rent a jackhammer depends on the length of time it is rented. Dependent variable: cost Independent variable: time

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Check It Out! Example 2b Identify the independent and dependent variable in the situation. Camryn buys p pounds of apples at $0.99 per pound. The cost of apples depends on the number of pounds bought. Dependent variable: cost Independent variable: pounds

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**An algebraic expression that defines a function is a function rule.**

If x is the independent variable and y is the dependent variable, then function notation for y is f(x), read “f of x,” where f names the function. When an equation in two variables describes a function, you can use function notation to write it.

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**The dependent variable is a function of the independent variable.**

y is a function of x. y = f (x) y = f(x)

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**Example 3A: Writing Functions**

Identify the independent and dependent variables. Write a rule in function notation for the situation. A math tutor charges $35 per hour. The amount a math tutor charges depends on number of hours. Dependent: charges Independent: hours Let h represent the number of hours of tutoring. The function for the amount a math tutor charges is f(h) = 35h.

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**Example 3B: Writing Functions**

Identify the independent and dependent variables. Write a rule in function notation for the situation. A fitness center charges a $100 initiation fee plus $40 per month. The total cost depends on the number of months, plus $100. Dependent: total cost Independent: number of months Let m represent the number of months The function for the amount the fitness center charges is f(m) = 40m

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Check It Out! Example 3a Identify the independent and dependent variables. Write a rule in function notation for the situation. Steven buys lettuce that costs $1.69/lb. The total cost depends on how many pounds of lettuce that Steven buys. Dependent: total cost Independent: pounds Let x represent the number of pounds Steven bought. The function for cost of the lettuce is f(x) = 1.69x.

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Check It Out! Example 3b Identify the independent and dependent variables. Write a rule in function notation for the situation. An amusement park charges a $6.00 parking fee plus $29.99 per person. The total cost depends on the number of persons in the car, plus $6. Dependent: total cost Independent: number of persons in the car Let x represent the number of persons in the car. The function for the total park cost is f(x) = 29.99x + 6.

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**You can think of a function as an input-output machine.**

x 2 You can think of a function as an input-output machine. 6 function f(x)=5x 5x 10 30 output

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**Example 4A: Evaluating Functions**

Evaluate the function for the given input values. For f(x) = 3x + 2, find f(x) when x = 7 and when x = –4. f(x) = 3(x) + 2 f(x) = 3(x) + 2 Substitute 7 for x. Substitute –4 for x. f(7) = 3(7) + 2 f(–4) = 3(–4) + 2 = Simplify. Simplify. = –12 + 2 = 23 = –10

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**Example 4B: Evaluating Functions**

Evaluate the function for the given input values. For g(t) = 1.5t – 5, find g(t) when t = 6 and when t = –2. g(t) = 1.5t – 5 g(t) = 1.5t – 5 g(6) = 1.5(6) – 5 g(–2) = 1.5(–2) – 5 = 9 – 5 = –3 – 5 = 4 = –8

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**Example 4C: Evaluating Functions**

Evaluate the function for the given input values. For , find h(r) when r = 600 and when r = –12. = 202 = –2

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Check It Out! Example 4a Evaluate the function for the given input values. For h(c) = 2c – 1, find h(c) when c = 1 and when c = –3. h(c) = 2c – 1 h(c) = 2c – 1 h(1) = 2(1) – 1 h(–3) = 2(–3) – 1 = 2 – 1 = –6 – 1 = 1 = –7

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Check It Out! Example 4b Evaluate each function for the given input values. For g(t) = , find g(t) when t = –24 and when t = 400. = –5 = 101

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When a function describes a real-world situation, every real number is not always reasonable for the domain and range. For example, a number representing the length of an object cannot be negative, and only whole numbers can represent a number of people.

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**Example 5: Finding the Reasonable Range and Domain of a Function**

Joe has enough money to buy 1, 2, or 3 DVDs at $15.00 each. Write a function to describe the situation. Find a reasonable domain and range of the function. Money spent is $ for each DVD. f(x) = $ • x If Joe buys x DVDs, he will spend f(x) = 15x dollars. Joe only has enough money to purchase 1, 2, or 3 DVDs. A reasonable domain is {1, 2, 3}.

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**A reasonable range for this situation is {$15, $30, $45}.**

Example 5 Continued Substitute the domain values into the function rule to find the range values. x 1 2 3 f(x) 15(1) = 15 15(2) = 30 15(3) = 45 A reasonable range for this situation is {$15, $30, $45}.

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Check It Out! Example 5 The settings on a space heater are the whole numbers from 0 to 3. The total of watts used for each setting is 500 times the setting number. Write a function rule to describe the number of watts used for each setting. Find a reasonable domain and range for the function. Number of watts used is times the setting #. watts f(x) = • x For each setting, the number of watts is f(x) = 500x watts.

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**A reasonable range for this situation is {0, 500, 1,000, 1,500} watts.**

Check It Out! Example 5 There are 4 possible settings 0, 1, 2, and 3, so a reasonable domain would be {0, 1, 2, 3}. Substitute these values into the function rule to find the range values. x f(x) 1 2 3 500(0) = 500(1) = 500 500(2) = 1,000 500(3) = 1,500 A reasonable range for this situation is {0, 500, 1,000, 1,500} watts.

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Lesson Quiz: Part I Identify the independent and dependent variables. Write a rule in function notation for each situation. 1. A buffet charges $8.95 per person. independent: number of people dependent: cost f(p) = 8.95p 2. A moving company charges $130 for weekly truck rental plus $1.50 per mile. independent: miles dependent: cost f(m) = m

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Lesson Quiz: Part II Evaluate each function for the given input values. 3. For g(t) = , find g(t) when t = 20 and when t = –12. g(20) = 2 g(–12) = –6 4. For f(x) = 6x – 1, find f(x) when x = 3.5 and when x = –5. f(3.5) = 20 f(–5) = –31

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Lesson Quiz: Part III Write a function to describe the situation. Find a reasonable domain and range for the function. 5. A theater can be rented for exactly 2, 3, or 4 hours. The cost is a $100 deposit plus $200 per hour. f(h) = 200h + 100 Domain: {2, 3, 4} Range: {$500, $700, $900}

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