# 10.E: Linear Equations (Optional Exercises)

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*Use the following information to answer the next three exercises*. A vacation resort rents SCUBA equipment to certified divers. The resort charges an up-front fee of $25 and another fee of $12.50 an hour.

Exercise 12.2.5

What are the dependent and independent variables?

**Answer**

dependent variable: fee amount; independent variable: time

Exercise 12.2.6

Find the equation that expresses the total fee in terms of the number of hours the equipment is rented.

Exercise 12.2.7

Graph the equation from Exercise.

**Answer**

*Use the following information to answer the next two exercises*. A credit card company charges $10 when a payment is late, and $5 a day each day the payment remains unpaid.

Exercise 12.2.8

Find the equation that expresses the total fee in terms of the number of days the payment is late.

Exercise 12.2.9

Graph the equation from Exercise.

**Answer**

Exercise 12.2.10

Is the equation \(y = 10 + 5x – 3x^{2}\) linear? Why or why not?

Exercise 12.2.11

Which of the following equations are linear?

- \(y = 6x + 8\)
- \(y + 7 = 3x\)
- \(y – x = 8x^{2}\)
- \(4y = 8\)

**Answer**

\(y = 6x + 8\), \(4y = 8\), and \(y + 7 = 3x\) are all linear equations.

Exercise 12.2.12

Does the graph show a linear equation? Why or why not?

Table contains real data for the first two decades of AIDS reporting.

Year |
# AIDS cases diagnosed |
# AIDS deaths |
---|---|---|

Pre-1981 | 91 | 29 |

1981 | 319 | 121 |

1982 | 1,170 | 453 |

1983 | 3,076 | 1,482 |

1984 | 6,240 | 3,466 |

1985 | 11,776 | 6,878 |

1986 | 19,032 | 11,987 |

1987 | 28,564 | 16,162 |

1988 | 35,447 | 20,868 |

1989 | 42,674 | 27,591 |

1990 | 48,634 | 31,335 |

1991 | 59,660 | 36,560 |

1992 | 78,530 | 41,055 |

1993 | 78,834 | 44,730 |

1994 | 71,874 | 49,095 |

1995 | 68,505 | 49,456 |

1996 | 59,347 | 38,510 |

1997 | 47,149 | 20,736 |

1998 | 38,393 | 19,005 |

1999 | 25,174 | 18,454 |

2000 | 25,522 | 17,347 |

2001 | 25,643 | 17,402 |

2002 | 26,464 | 16,371 |

Total |
802,118 |
489,093 |

Exercise 12.2.13

Use the columns "year" and "# AIDS cases diagnosed. Why is “year” the independent variable and “# AIDS cases diagnosed.” the dependent variable (instead of the reverse)?

**Answer**

The number of AIDS cases depends on the year. Therefore, year becomes the independent variable and the number of AIDS cases is the dependent variable.

*Use the following information to answer the next two exercises*. A specialty cleaning company charges an equipment fee and an hourly labor fee. A linear equation that expresses the total amount of the fee the company charges for each session is \(y = 50 + 100x\).

Exercise 12.2.14

What are the independent and dependent variables?

Exercise 12.2.15

What is the *y*-intercept and what is the slope? Interpret them using complete sentences.

**Answer**

The \(y\)-intercept is 50 (\(a = 50\)). At the start of the cleaning, the company charges a one-time fee of $50 (this is when \(x = 0\)). The slope is 100 (\(b = 100\)). For each session, the company charges $100 for each hour they clean.

*Use the following information to answer the next three questions*. Due to erosion, a river shoreline is losing several thousand pounds of soil each year. A linear equation that expresses the total amount of soil lost per year is \(y = 12,000x\).

Exercise 12.2.16

What are the independent and dependent variables?

Exercise 12.2.17

How many pounds of soil does the shoreline lose in a year?

**Answer**

12,000 pounds of soil

Exercise 12.2.18

What is the \(y\)-intercept? Interpret its meaning.

*Use the following information to answer the next two exercises*. The price of a single issue of stock can fluctuate throughout the day. A linear equation that represents the price of stock for Shipment Express is \(y = 15 – 1.5x\) where \(x\) is the number of hours passed in an eight-hour day of trading.

Exercise 12.2.19

What are the slope and *y*-intercept? Interpret their meaning.

**Answer**

The slope is -1.5 (\(b = -1.5\)). This means the stock is losing value at a rate of $1.50 per hour. The \(y\)-intercept is $15 (\(a = 15\)). This means the price of stock before the trading day was $15.

Exercise 12.2.19

If you owned this stock, would you want a positive or negative slope? Why?