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Section 6.5 Interlude: Linear vs. exponential models in point-slope form

Subsection Movie ticket prices

Calliope read in Section 5.4 that in 1995, the average price of a movie ticket was $4.35, and in 2011 the average price of a movie ticket was $7.93.

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She wonders whether or a linear or exponential model would do a better job of predicting the average price of a movie ticket

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in 2026, which is estimated to be $12.87.

Subsection Linear model for movie ticket prices

Normally, Calliope would measure time in years since 1995. But she recently learned about the point-slope equation of a line, she is wondering what it would look like if she just used \(Y\) for the actual year instead. That is, the variables would be
\begin{align*} Y \amp= \text{ time (actual year) }\sim \text{ indep}\\ T \amp= \text{ average price of a movie ticket (\$) }\sim \text{ dep} \end{align*}
To write a linear model, Calliope starts by calculating the slope which is
\begin{equation*} \frac{\$7.93-\$4.35}{2011-1995}= \frac{\$3.58}{16 \text{ years}}= 0.22375 \approx \$0.224/\text{year} \end{equation*}
She then shows off her mad point-slope equation skills and writes the linear equation
\begin{equation*} T = 4.35 + 0.224(Y - 1995) \end{equation*}
Calliope checks her linear equation for the year 2011
\begin{equation*} T = 4.35 + 0.224(2011 - 1995) = 7.934 \approx \$7.93 \end{equation*}
Since she rounded off to get the slope of 0.224, she needed to round off the answer as well.
In 2026, we have \(Y=2026\) and so this linear model predicts the average movie ticket price would be
\begin{equation*} 4.35 + 0.224(2026 - 1995) = 11.284 \approx \$11.29 \end{equation*}

Subsection Exponential model for ticket prices

To write an exponential model, Calliope starts by calculating the growth factor which, by the Growth Factor Formula is
\begin{equation*} g = \sqrt[2011-1995]{\frac{7.93}{4.35}}= \sqrt[16]{\frac{7.93}{4.35}}= 1.0382429616 \approx 1.0382 \end{equation*}
She then writes the exponential equation using this growth factor and the point.
\begin{equation*} T = 4.35(1.0382)^{Y-1995} \end{equation*}
Notice that because Calliope is using the actual year, the equation needs to use \(Y-1995\) in the exponent.
Calliope checks her exponential equation for the year 2011
\begin{equation*} T = 4.35(1.0382)^{2011-1995}= 4.35 \times 1.0382 \land (2011-1995) = 7.9247\ldots \approx \$7.92 \end{equation*}
Since she rounded off to get the growth factor of 1.0382, this answer is slightly off as well.
In 2026, we have \(Y=2026\) and so this exponential model predicts the average movie ticket price would be
\begin{equation*} 4.35(1.0382)^{2026-1995}=4.35 \times 1.0382 \land (2026-1995)=13.9059\ldots \approx \$13.90 \end{equation*}

Subsection Comparing linear vs. exponential

Both the linear and exponential model did fairly well at predicting today’s prices. The linear model estimate of $11.29 was \(12.87-11.29 = \$1.58\) lower than the actual average price of a movie ticket but the exponential model estimate of $13.90 was about \(13.90-12.87 = \$1.03\) higher. The exponential is probably a slightly better estimate, but there’s some rounding error here so Calliope decides they are both quite close.

Subsection Do you know …

  1. How to write the equation of a line using the actual year?
  2. How to write an exponential equation of a line using the actual year?
If you’re not sure, work the rest of the exercises and then return to these questions. Or, ask your instructor or a classmate for help.

Exercises Exercises

Exercises 1-4 are available in a separate workbook format.

1.

My parents bought the house I grew up in for $35,000 in 1964 and sold it 40 years later (in 2004) for $342,000. True story. (It was before the housing bubble burst.)

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(a)

Write a linear and exponential model and compare the house’s value in 2026 according to each model. Don’t forget to name the variables, including units. Use the actual year.

(b)

The house is valued at $687,000 in 2026.
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How well did your linear and exponential models predict this current value?

2.

The number of manufacturing jobs in the state has been declining for decades. In 1970, there were 1.2 million such jobs in the state but by 2010 there were only 0.6 million such jobs.

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(a)

Write a linear and exponential model and compare the predicted number of manufacturing jobs in 2025 according to each model. Use the actual year.

(b)

Compare to the actual number, which is reported to be 12.5 million in 2025.
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What can you say about manufacturing jobs in the late 1900s/early 2000s versus now?

3.

The number of asthma sufferers worldwide in 1990 was 84 million and 130 million in 2001.

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(a)

Compare what the linear and exponential models project for the year 2023 and 2030. Use the actual year.

(b)

Compare to the actual number which was reported to be 363 million in 2023.
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Which model is closer?

4.

Sales of hybrid cars in the United States have continued to increase. In 1999, 17 (yes, seventeen) hybrid cars were sold. By 2002, that number was up to 34,521 hybrid cars sold. Compare what the linear and exponential models projected for hybrid car sales in the year 2025 and compare to the actual number which is reported to be 2.05 million in 2025, which doesn’t even include electric cars.

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Use the actual year.

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