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College Algebra

5.8 Modeling Using Variation

College Algebra5.8 Modeling Using Variation
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  1. Preface
  2. 1 Prerequisites
    1. Introduction to Prerequisites
    2. 1.1 Real Numbers: Algebra Essentials
    3. 1.2 Exponents and Scientific Notation
    4. 1.3 Radicals and Rational Exponents
    5. 1.4 Polynomials
    6. 1.5 Factoring Polynomials
    7. 1.6 Rational Expressions
    8. Key Terms
    9. Key Equations
    10. Key Concepts
    11. Review Exercises
    12. Practice Test
  3. 2 Equations and Inequalities
    1. Introduction to Equations and Inequalities
    2. 2.1 The Rectangular Coordinate Systems and Graphs
    3. 2.2 Linear Equations in One Variable
    4. 2.3 Models and Applications
    5. 2.4 Complex Numbers
    6. 2.5 Quadratic Equations
    7. 2.6 Other Types of Equations
    8. 2.7 Linear Inequalities and Absolute Value Inequalities
    9. Key Terms
    10. Key Equations
    11. Key Concepts
    12. Review Exercises
    13. Practice Test
  4. 3 Functions
    1. Introduction to Functions
    2. 3.1 Functions and Function Notation
    3. 3.2 Domain and Range
    4. 3.3 Rates of Change and Behavior of Graphs
    5. 3.4 Composition of Functions
    6. 3.5 Transformation of Functions
    7. 3.6 Absolute Value Functions
    8. 3.7 Inverse Functions
    9. Key Terms
    10. Key Equations
    11. Key Concepts
    12. Review Exercises
    13. Practice Test
  5. 4 Linear Functions
    1. Introduction to Linear Functions
    2. 4.1 Linear Functions
    3. 4.2 Modeling with Linear Functions
    4. 4.3 Fitting Linear Models to Data
    5. Key Terms
    6. Key Concepts
    7. Review Exercises
    8. Practice Test
  6. 5 Polynomial and Rational Functions
    1. Introduction to Polynomial and Rational Functions
    2. 5.1 Quadratic Functions
    3. 5.2 Power Functions and Polynomial Functions
    4. 5.3 Graphs of Polynomial Functions
    5. 5.4 Dividing Polynomials
    6. 5.5 Zeros of Polynomial Functions
    7. 5.6 Rational Functions
    8. 5.7 Inverses and Radical Functions
    9. 5.8 Modeling Using Variation
    10. Key Terms
    11. Key Equations
    12. Key Concepts
    13. Review Exercises
    14. Practice Test
  7. 6 Exponential and Logarithmic Functions
    1. Introduction to Exponential and Logarithmic Functions
    2. 6.1 Exponential Functions
    3. 6.2 Graphs of Exponential Functions
    4. 6.3 Logarithmic Functions
    5. 6.4 Graphs of Logarithmic Functions
    6. 6.5 Logarithmic Properties
    7. 6.6 Exponential and Logarithmic Equations
    8. 6.7 Exponential and Logarithmic Models
    9. 6.8 Fitting Exponential Models to Data
    10. Key Terms
    11. Key Equations
    12. Key Concepts
    13. Review Exercises
    14. Practice Test
  8. 7 Systems of Equations and Inequalities
    1. Introduction to Systems of Equations and Inequalities
    2. 7.1 Systems of Linear Equations: Two Variables
    3. 7.2 Systems of Linear Equations: Three Variables
    4. 7.3 Systems of Nonlinear Equations and Inequalities: Two Variables
    5. 7.4 Partial Fractions
    6. 7.5 Matrices and Matrix Operations
    7. 7.6 Solving Systems with Gaussian Elimination
    8. 7.7 Solving Systems with Inverses
    9. 7.8 Solving Systems with Cramer's Rule
    10. Key Terms
    11. Key Equations
    12. Key Concepts
    13. Review Exercises
    14. Practice Test
  9. 8 Analytic Geometry
    1. Introduction to Analytic Geometry
    2. 8.1 The Ellipse
    3. 8.2 The Hyperbola
    4. 8.3 The Parabola
    5. 8.4 Rotation of Axes
    6. 8.5 Conic Sections in Polar Coordinates
    7. Key Terms
    8. Key Equations
    9. Key Concepts
    10. Review Exercises
    11. Practice Test
  10. 9 Sequences, Probability, and Counting Theory
    1. Introduction to Sequences, Probability and Counting Theory
    2. 9.1 Sequences and Their Notations
    3. 9.2 Arithmetic Sequences
    4. 9.3 Geometric Sequences
    5. 9.4 Series and Their Notations
    6. 9.5 Counting Principles
    7. 9.6 Binomial Theorem
    8. 9.7 Probability
    9. Key Terms
    10. Key Equations
    11. Key Concepts
    12. Review Exercises
    13. Practice Test
  11. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
    7. Chapter 7
    8. Chapter 8
    9. Chapter 9
  12. Index

Learning Objectives

In this section, you will:
  • Solve direct variation problems.
  • Solve inverse variation problems.
  • Solve problems involving joint variation.

A used-car company has just offered their best candidate, Nicole, a position in sales. The position offers 16% commission on her sales. Her earnings depend on the amount of her sales. For instance, if she sells a vehicle for $4,600, she will earn $736. She wants to evaluate the offer, but she is not sure how. In this section, we will look at relationships, such as this one, between earnings, sales, and commission rate.

Solving Direct Variation Problems

In the example above, Nicole’s earnings can be found by multiplying her sales by her commission. The formula e=0.16s e=0.16stells us her earnings, e, e,come from the product of 0.16, her commission, and the sale price of the vehicle. If we create a table, we observe that as the sales price increases, the earnings increase as well, which should be intuitive. See Table 1.

s s, sales price e=0.16s e=0.16s Interpretation
$4,600 e=0.16( 4,600 )=736 e=0.16( 4,600 )=736 A sale of a $4,600 vehicle results in $736 earnings.
$9,200 e=0.16( 9,200 )=1,472 e=0.16( 9,200 )=1,472 A sale of a $9,200 vehicle results in $1472 earnings.
$18,400 e=0.16( 18,400 )=2,944 e=0.16( 18,400 )=2,944 A sale of a $18,400 vehicle results in $2944 earnings.
Table 1

Notice that earnings are a multiple of sales. As sales increase, earnings increase in a predictable way. Double the sales of the vehicle from $4,600 to $9,200, and we double the earnings from $736 to $1,472. As the input increases, the output increases as a multiple of the input. A relationship in which one quantity is a constant multiplied by another quantity is called direct variation. Each variable in this type of relationship varies directly with the other.

Figure 1 represents the data for Nicole’s potential earnings. We say that earnings vary directly with the sales price of the car. The formula y=k x n y=k x n is used for direct variation. The value k kis a nonzero constant greater than zero and is called the constant of variation. In this case, k=0.16 k=0.16and n=1. n=1.We saw functions like this one when we discussed power functions.

Graph of y=(0.16)x where the horizontal axis is labeled, “s, Sales Price in Dollars”, and the vertical axis is labeled, “e, Earnings, $”.
Figure 1

Direct Variation

If xandy xandy are related by an equation of the form

y=k x n y=k x n

then we say that the relationship is direct variation and y y varies directly with, or is proportional to, the nth nthpower of x. x. In direct variation relationships, there is a nonzero constant ratio k= y x n , k= y x n ,where k kis called the constant of variation, which help defines the relationship between the variables.

How To

Given a description of a direct variation problem, solve for an unknown.

  1. Identify the input, x, x, and the output, y. y.
  2. Determine the constant of variation. You may need to divide y yby the specified power of x xto determine the constant of variation.
  3. Use the constant of variation to write an equation for the relationship.
  4. Substitute known values into the equation to find the unknown.

Example 1

Solving a Direct Variation Problem

The quantity y yvaries directly with the cube of x. x.If y=25 y=25when x=2, x=2,find y ywhen x xis 6.

Analysis

The graph of this equation is a simple cubic, as shown in Figure 2.

Graph of y=25/8(x^3) with the labeled points (2, 25) and (6, 675).
Figure 2

Q&A

Do the graphs of all direct variation equations look like Example 1?

No. Direct variation equations are power functions—they may be linear, quadratic, cubic, quartic, radical, etc. But all of the graphs pass through ( 0,0 ). ( 0,0 ).

Try It #1

The quantity y yvaries directly with the square of x. x.If y=24 y=24when x=3, x=3,find y ywhen x xis 4.

Solving Inverse Variation Problems

Water temperature in an ocean varies inversely to the water’s depth. The formula T= 14,000 d T= 14,000 d gives us the temperature in degrees Fahrenheit at a depth in feet below Earth’s surface. Consider the Atlantic Ocean, which covers 22% of Earth’s surface. At a certain location, at the depth of 500 feet, the temperature may be 28°F.

If we create Table 2, we observe that, as the depth increases, the water temperature decreases.

d, d, depth T= 14,000 d T= 14,000 d Interpretation
500 ft 14,000 500 =28 14,000 500 =28 At a depth of 500 ft, the water temperature is 28° F.
1000 ft 14,000 1000 =14 14,000 1000 =14 At a depth of 1,000 ft, the water temperature is 14° F.
2000 ft 14,000 2000 =7 14,000 2000 =7 At a depth of 2,000 ft, the water temperature is 7° F.
Table 2

We notice in the relationship between these variables that, as one quantity increases, the other decreases. The two quantities are said to be inversely proportional and each term varies inversely with the other. Inversely proportional relationships are also called inverse variations.

For our example, Figure 3 depicts the inverse variation. We say the water temperature varies inversely with the depth of the water because, as the depth increases, the temperature decreases. The formula y= k x y= k x for inverse variation in this case uses k=14,000. k=14,000.

Graph of y=(14000)/x where the horizontal axis is labeled, “Depth, d (ft)”, and the vertical axis is labeled, “Temperature, T (Degrees Fahrenheit)”.
Figure 3

Inverse Variation

If x xand y yare related by an equation of the form

y= k x n y= k x n

where k kis a nonzero constant, then we say that y y varies inversely with the nth nthpower of x. x.In inversely proportional relationships, or inverse variations, there is a constant multiple k= x n y. k= x n y.

Example 2

Writing a Formula for an Inversely Proportional Relationship

A tourist plans to drive 100 miles. Find a formula for the time the trip will take as a function of the speed the tourist drives.

How To

Given a description of an indirect variation problem, solve for an unknown.

  1. Identify the input, x, x,and the output, y. y.
  2. Determine the constant of variation. You may need to multiply y yby the specified power of x xto determine the constant of variation.
  3. Use the constant of variation to write an equation for the relationship.
  4. Substitute known values into the equation to find the unknown.

Example 3

Solving an Inverse Variation Problem

A quantity y yvaries inversely with the cube of x. x.If y=25 y=25when x=2, x=2,find y ywhen x xis 6.

Analysis

The graph of this equation is a rational function, as shown in Figure 4.

Graph of y=25/(x^3) with the labeled points (2, 25) and (6, 25/27).
Figure 4
Try It #2

A quantity y yvaries inversely with the square of x. x.If y=8 y=8when x=3, x=3,find y ywhen x xis 4.

Solving Problems Involving Joint Variation

Many situations are more complicated than a basic direct variation or inverse variation model. One variable often depends on multiple other variables. When a variable is dependent on the product or quotient of two or more variables, this is called joint variation. For example, the cost of busing students for each school trip varies with the number of students attending and the distance from the school. The variable c, c, cost, varies jointly with the number of students, n, n, and the distance, d. d.

Joint Variation

Joint variation occurs when a variable varies directly or inversely with multiple variables.

For instance, if x xvaries directly with both y yand z, z, we have x=kyz. x=kyz.If x xvaries directly with y yand inversely with z, z,we have x= ky z . x= ky z .Notice that we only use one constant in a joint variation equation.

Example 4

Solving Problems Involving Joint Variation

A quantity x xvaries directly with the square of y yand inversely with the cube root of z. z.If x=6 x=6when y=2 y=2 and z=8, z=8,find x xwhen y=1 y=1 and z=27. z=27.

Try It #3

A quantity x xvaries directly with the square of y yand inversely with z. z.If x=40 x=40when y=4 y=4and z=2, z=2,find x xwhen y=10 y=10 and z=25. z=25.

Media

Access these online resources for additional instruction and practice with direct and inverse variation.

Visit this website for additional practice questions from Learningpod.

5.8 Section Exercises

Verbal

1.

What is true of the appearance of graphs that reflect a direct variation between two variables?

2.

If two variables vary inversely, what will an equation representing their relationship look like?

3.

Is there a limit to the number of variables that can vary jointly? Explain.

Algebraic

For the following exercises, write an equation describing the relationship of the given variables.

4.

y yvaries directly as x xand when x=6, y=12. x=6, y=12.

5.

y yvaries directly as the square of x xand when x=4, y=80.  x=4, y=80. 

6.

y yvaries directly as the square root of x xand when x=36, y=24. x=36, y=24.

7.

y yvaries directly as the cube of x xand when x=36,y=24. x=36,y=24.

8.

y yvaries directly as the cube root of x xand when x=27, y=15. x=27, y=15.

9.

y yvaries directly as the fourth power of x xand when x=1, y=6. x=1, y=6.

10.

y yvaries inversely as x xand when x=4, y=2. x=4, y=2.

11.

y yvaries inversely as the square of x xand when x=3, y=2. x=3, y=2.

12.

y yvaries inversely as the cube of x xand when x=2, y=5. x=2, y=5.

13.

y yvaries inversely as the fourth power of x xand when x=3, y=1. x=3, y=1.

14.

y yvaries inversely as the square root of x xand when x=25, y=3. x=25, y=3.

15.

y yvaries inversely as the cube root of x xand when x=64, y=5. x=64, y=5.

16.

y yvaries jointly with x xand z zand when x=2 x=2and z=3, y=36. z=3, y=36.

17.

y yvaries jointly as x, z, x, z,and w wand when x=1, z=2, w=5, x=1, z=2, w=5,then y=100. y=100.

18.

y yvaries jointly as the square of x xand the square of z zand when x=3 x=3and z=4, z=4,then y=72. y=72.

19.

y yvaries jointly as x xand the square root of z zand when x=2 x=2and z=25, z=25,then y=100. y=100.

20.

y yvaries jointly as the square of x xthe cube of z zand the square root of W. W. When x=1,z=2, x=1,z=2,and w=36, w=36,then y=48. y=48.

21.

y y varies jointly as x and z x and zand inversely as w. w. When x=3, z=5, x=3, z=5,and w=6, w=6,then y=10. y=10.

22.

y yvaries jointly as the square of x xand the square root of z zand inversely as the cube of w.  w. When x=3,z=4, x=3,z=4,and w=3, w=3,then y=6. y=6.

23.

y yvaries jointly as x xand z zand inversely as the square root of w wand the square of t . t .When x=3,z=1,w=25, x=3,z=1,w=25,and t=2, t=2,then y=6. y=6.

Numeric

For the following exercises, use the given information to find the unknown value.

24.

y yvaries directly as x. x. When x=3, x=3,then y=12. y=12.Find y ywneh x=20. x=20.

25.

y yvaries directly as the square of x. x. When x=2, x=2,then y=16. y=16.Find y ywhen x=8. x=8.

26.

y yvaries directly as the cube of x. x. When x=3, x=3,then y=5.  y=5. Find y ywhen x=4. x=4.

27.

y yvaries directly as the square root of x. x. When x=16, x=16,then y=4. y=4.Find y ywhen x=36. x=36.

28.

y yvaries directly as the cube root of x. x. When x=125, x=125,then y=15. y=15.Find y ywhen x=1,000. x=1,000.

29.

y yvaries inversely with x. x. When x=3, x=3,then y=2. y=2.Find y ywhen x=1. x=1.

30.

y yvaries inversely with the square of x. x. When x=4, x=4,then y=3. y=3.Find y ywhen x=2. x=2.

31.

y yvaries inversely with the cube of x. x. When x=3, x=3,then y=1. y=1.Find y ywhen x=1. x=1.

32.

y yvaries inversely with the square root of x. x. When x=64, x=64,then y=12. y=12. Find y ywhen x=36. x=36.

33.

y yvaries inversely with the cube root of x. x. When x=27, x=27,then y=5. y=5. Find y ywhen x=125. x=125.

34.

y yvaries jointly as x and z. x and z.When x=4 x=4and z=2, z=2,then y=16. y=16.Find y ywhen x=3 x=3and z=3. z=3.

35.

y yvaries jointly as x, z, and w. x, z, and w.When x=2, x=2, z=1, z=1,and w=12, w=12,then y=72. y=72.Find y ywhen x=1, x=1, z=2, z=2, and w=3. w=3.

36.

y yvaries jointly as x xand the square of z. z.When x=2 x=2and z=4, z=4,then y=144. y=144.Find y ywhen x=4 x=4and z=5. z=5.

37.

y yvaries jointly as the square of x xand the square root of z. z. When x=2 x=2and z=9, z=9,then y=24. y=24.Find y ywhen x=3 x=3and z=25. z=25.

38.

y yvaries jointly as x xand z zand inversely as w. w. When x=5, x=5, z=2,  z=2, and  w=20,  w=20,then y=4. y=4.Find y ywhen x=3 x=3and z=8,  z=8, and  w=48.  w=48.

39.

y yvaries jointly as the square of x xand the cube of z zand inversely as the square root of w.  w. When x=2, x=2, z=2, z=2,and w=64, w=64,then y=12. y=12.Find y ywhen x=1, x=1, z=3, z=3,and  w=4.  w=4.

40.

y yvaries jointly as the square of x xand of z zand inversely as the square root of w wand of t . t .When x=2, x=2, z=3, z=3, w=16, w=16,and t=3, t=3,then y=1. y=1.Find y ywhen x=3, x=3, z=2, z=2,  w=36,  w=36,and t=5. t=5.

Technology

For the following exercises, use a calculator to graph the equation implied by the given variation.

41.

y yvaries directly with the square of x xand when x=2, y=3. x=2, y=3.

42.

y yvaries directly as the cube of x xand when x=2, y=4. x=2, y=4.

43.

y yvaries directly as the square root of x xand when x=36, y=2. x=36, y=2.

44.

y yvaries inversely with x xand when x=6, y=2. x=6, y=2.

45.

y yvaries inversely as the square of x xand when x=1, y=4. x=1, y=4.

Extensions

For the following exercises, use Kepler’s Law, which states that the square of the time, T, T,required for a planet to orbit the Sun varies directly with the cube of the mean distance, a, a,that the planet is from the Sun.

46.

Using Earth’s time of 1 year and mean distance of 93 million miles, find the equation relating T T and a. a.

47.

Use the result from the previous exercise to determine the time required for Mars to orbit the Sun if its mean distance is 142 million miles.

48.

Using Earth’s distance of 150 million kilometers, find the equation relating T Tand a. a.

49.

Use the result from the previous exercise to determine the time required for Venus to orbit the Sun if its mean distance is 108 million kilometers.

50.

Using Earth’s distance of 1 astronomical unit (A.U.), determine the time for Saturn to orbit the Sun if its mean distance is 9.54 A.U.

Real-World Applications

For the following exercises, use the given information to answer the questions.

51.

The distance s sthat an object falls varies directly with the square of the time, t, t, of the fall. If an object falls 16 feet in one second, how long for it to fall 144 feet?

52.

The velocity v vof a falling object varies directly to the time, t, t, of the fall. If after 2 seconds, the velocity of the object is 64 feet per second, what is the velocity after 5 seconds?

53.

The rate of vibration of a string under constant tension varies inversely with the length of the string. If a string is 24 inches long and vibrates 128 times per second, what is the length of a string that vibrates 64 times per second?

54.

The volume of a gas held at constant temperature varies indirectly as the pressure of the gas. If the volume of a gas is 1200 cubic centimeters when the pressure is 200 millimeters of mercury, what is the volume when the pressure is 300 millimeters of mercury?

55.

The weight of an object above the surface of Earth varies inversely with the square of the distance from the center of Earth. If a body weighs 50 pounds when it is 3960 miles from Earth’s center, what would it weigh it were 3970 miles from Earth’s center?

56.

The intensity of light measured in foot-candles varies inversely with the square of the distance from the light source. Suppose the intensity of a light bulb is 0.08 foot-candles at a distance of 3 meters. Find the intensity level at 8 meters.

57.

The current in a circuit varies inversely with its resistance measured in ohms. When the current in a circuit is 40 amperes, the resistance is 10 ohms. Find the current if the resistance is 12 ohms.

58.

The force exerted by the wind on a plane surface varies jointly with the square of the velocity of the wind and with the area of the plane surface. If the area of the surface is 40 square feet surface and the wind velocity is 20 miles per hour, the resulting force is 15 pounds. Find the force on a surface of 65 square feet with a velocity of 30 miles per hour.

59.

The horsepower (hp) that a shaft can safely transmit varies jointly with its speed (in revolutions per minute (rpm) and the cube of the diameter. If the shaft of a certain material 3 inches in diameter can transmit 45 hp at 100 rpm, what must the diameter be in order to transmit 60 hp at 150 rpm?

60.

The kinetic energy K Kof a moving object varies jointly with its mass m mand the square of its velocity v. v. If an object weighing 40 kilograms with a velocity of 15 meters per second has a kinetic energy of 1000 joules, find the kinetic energy if the velocity is increased to 20 meters per second.

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