Fit Functions to Real-World Data

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Statistics › Fit Functions to Real-World Data

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1

A student uses a calculator’s regression feature to fit a model to the following data relating the number of weeks since planting ($w$) to the height of a plant ($h$, in cm).

Data: $(1, 4.5), (2, 7.0), (3, 9.5), (4, 12.0), (5, 14.5), (6, 17.0)$

Which function is most reasonable for modeling this data?

$h=w^2+3$

$h=2.5w+2.0$

$h=4.5(1.25)^w$

$h=\dfrac{20}{w}$

Explanation

Fitting functions to bivariate data involves identifying patterns, such as constant first differences for linear models. The key cue is the uniform increase of 2.5 cm in height per week, seen in differences like 7.0 - 4.5 = 2.5 and so on. Applying this to the points (1,4.5) to (6,17), the data forms a straight line with slope 2.5 and intercept 2.0. The linear function h=2.5w+2.0 is reasonable for steady plant growth. No prediction needed, but it would forecast height accurately. A common misconception is choosing exponential for increases, but constant differences rule out varying ratios. Always check first differences, second differences, and ratios for model selection.

2

A savings account grows by a fixed percent each year. The balance is recorded at the end of each year.

Which type of function best models the relationship between years and account balance?

Years (x): 0, 1, 2, 3, 4, 5

Balance ($y): 1000, 1060, 1123.6, 1191.0, 1262.5, 1338.2

Quadratic

Step function

Exponential

Linear

Explanation

For bivariate data, exponential models apply when there's constant proportional growth, like in compound interest. The key cue is the constant ratio of 1.06 in balances: 1060/1000 = 1.06, 1123.6/1060 ≈ 1.06, continuing similarly. This multiplicative pattern fits the annual percentage growth. An exponential function is reasonable for savings accounts with fixed interest rates. Commonly, people confuse this with linear addition, but ratios reveal the compounding. Compute both differences and ratios as a strategy to identify the appropriate model.

3

A scientist cools a beaker of hot water in a room with constant temperature. Over short time intervals, the temperature readings are recorded as shown.

Which type of function is most reasonable for modeling the relationship between time and temperature over this time period?

Time (min): 0, 5, 10, 15, 20, 25

Temperature (°C): 90, 74, 62, 53, 46, 41

Quadratic

Exponential

Linear

Direct variation

Explanation

Modeling bivariate data often involves recognizing exponential decay when changes slow over time, as in cooling processes. The key cue is the decreasing differences in temperature (-16, -12, -9, -7, -5), and approximating constant ratios in (T - room temp), around 0.75 per 5 minutes assuming room temperature near 25°C. This pattern fits the data, showing the temperature approaching ambient levels asymptotically. An exponential function is reasonable for Newton's law of cooling over short intervals. A common misconception is fitting linear models to decays, but linear requires constant differences. To transfer this, compute differences and check for proportional changes before modeling.

4

A soccer player practices free kicks. The height of the ball is recorded at different times after it is kicked. The data are shown below.

Which type of function best models the relationship between time and height?

Time (s): 0, 1, 2, 3, 4, 5

Height (m): 1, 6, 9, 10, 9, 6

Linear

Quadratic

Exponential

Inverse variation

Explanation

To fit a function to bivariate data, we examine the shape, such as a parabolic curve with a turning point for quadratic models. The key cue is the height increasing to a maximum and then decreasing symmetrically: 1 to 10 and back down to 6. This curved behavior with a peak at 10 meters aligns with projectile motion under gravity. A quadratic function is reasonable because it captures the acceleration due to gravity affecting the ball's height over time. People often confuse this with linear models, but linear lacks the turning point seen here. As a transfer strategy, inspect for curved patterns or compute second differences after checking first differences and ratios.

5

A farmer tracks the area of a circular oil spill on a pond as it spreads. The radius increases by about 0.5 meters each minute, and the measured area is recorded.

Which type of function is most reasonable for modeling the relationship between time and area?

Time (min): 0, 1, 2, 3, 4, 5

Area (m$^2$): 3.1, 7.1, 12.6, 19.6, 28.3, 38.5

Quadratic

Inverse

Exponential

Linear

Explanation

Selecting a model for bivariate data involves recognizing quadratic relationships when second differences are roughly constant, as in area growth. The key cue is the increasing first differences (4, 5.5, 7, 8.7, 10.2) and near-constant second differences around 1.5, stemming from A = $π(r)^2$ with linear radius increase. This quadratic expansion applies to the circular spill's area over time. A quadratic function is reasonable for squared relationships in spreading phenomena. A misconception is assuming exponential for any growth, but here it's polynomial. Always check successive differences and ratios to determine the best fit.

6

A ball is rolled up a ramp and then comes back down. Its distance from the start is measured at equal time intervals. The data are shown.

Which type of function is most reasonable for modeling the relationship between time and distance?

Time (s): 0, 1, 2, 3, 4, 5

Distance (m): 0, 3, 5, 6, 5, 3

Absolute value

Quadratic

Linear

Exponential

Explanation

Fitting functions to data includes identifying quadratic models for parabolic shapes with a turning point. The key cue is the distance increasing to 6 meters and then decreasing symmetrically back to 3, indicating a peak. This behavior matches the ball's motion up and down the ramp under gravity. A quadratic function is reasonable as it models the acceleration and deceleration. A misconception is seeing symmetry as absolute value, but quadratic better fits the smooth curve. Check for turning points and second differences after initial differences and ratios for model selection.

7

A car rental company charges a base fee plus a constant cost per mile. The total cost is recorded for several trips.

Miles driven: 0, 25, 50, 75, 100, 125

Total cost ($): 18, 30.5, 43, 55.5, 68, 80.5

Which type of function best models the relationship between miles driven and total cost?

Linear

Quadratic

Power function

Exponential

Explanation

To choose a function for data, look for constant differences indicating linear, constant ratios for exponential, or turning points for quadratic. Here, the first differences in cost are 30.5-18=12.5, 43-30.5=12.5, 55.5-43=12.5, 68-55.5=12.5, 80.5-68=12.5, constant for every 25 miles. This reflects a fixed base fee plus per-mile charge, a linear pattern. Linear is suitable for additive cost structures like this. A common error is confusing it with exponential if focusing on total growth, but differences are constant while ratios decrease (e.g., 30.5/18≈1.69, 43/30.5≈1.41). Verify by calculating differences and ratios to confirm linearity.

8

A savings account balance grows by a consistent percent each year. The balance (in dollars) is recorded below.

Year: 0, 1, 2, 3, 4, 5

Balance ($): 1200, 1260, 1323, 1389, 1458, 1531

Which type of function best models the relationship between year and balance?

Cubic

Quadratic

Exponential

Linear

Explanation

Selecting a model for bivariate data requires spotting constant first differences for linear, constant ratios for exponential, or curved behaviors for quadratic. The key indicator here is the nearly constant ratios: 1260/1200=1.05, 1323/1260=1.05, 1389/1323≈1.05, 1458/1389≈1.05, 1531/1458≈1.05, pointing to exponential growth. This fits the consistent percentage increase in a savings account with compound interest. Exponential is the best choice as it captures multiplicative growth over time. Commonly, people assume linear due to steady increases (60, 63, 66, 69, 73), but ratios confirm exponential. Check both differences and ratios to avoid confusing additive and multiplicative patterns.

9

A streaming service tracked the total number of subscribers (in thousands) over time. The data are shown below.

Which type of function best models the relationship between time (months) and subscribers?

Months since launch: 0, 1, 2, 3, 4, 5

Subscribers (thousands): 20, 26, 34, 44, 57, 74

Quadratic

Linear

Exponential

None of these

Explanation

When selecting an appropriate model for bivariate data, we look for patterns like constant differences for linear functions, constant ratios for exponential functions, or curved behavior with constant second differences for quadratic functions. The key cue here is checking for constant ratios in the subscriber counts over time. Calculating the ratios: 26/20 = 1.3, 34/26 ≈ 1.307, 44/34 ≈ 1.294, 57/44 ≈ 1.295, and 74/57 ≈ 1.298, which are approximately constant around 1.3. This pattern suggests an exponential function is reasonable because it captures the multiplicative growth typical of subscriber increases through word-of-mouth or viral effects. A common misconception is assuming linear growth due to increasing differences (6, 8, 10, 13, 17), but the consistent ratios indicate exponential instead. To apply this strategy more broadly, always check first differences for linearity and ratios for exponentiality before selecting a model.

10

A cyclist records the total distance traveled during a steady ride. The data are shown below.

Time (minutes): 0, 10, 20, 30, 40, 50

Distance (miles): 0, 3, 6, 9, 12, 15

Which type of function best models the relationship between time and distance?

Exponential

Linear

Logarithmic

Quadratic

Explanation

Fitting a function to bivariate data involves identifying patterns such as constant first differences for linear models, constant ratios for exponential, or turning points for quadratic. The key cue in this data is the constant first differences in distance: 3-0=3, 6-3=3, 9-6=3, 12-9=3, and 15-12=3, all equal to 3 miles every 10 minutes. This consistent increase applies directly to the steady ride, indicating a linear relationship between time and distance. A linear model is reasonable as it reflects a constant speed without acceleration or deceleration. People often confuse this with exponential if they miscalculate ratios, but the differences are constant while ratios decrease (e.g., 3/0 undefined, then 6/3=2, but pattern doesn't hold). Remember to verify by computing differences and ratios to distinguish linear from exponential growth.

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