In another MathFAQ, I examined how we can find the equation of a line from two data points. In this post I want to look at a closely related problem where we find the equation of the line from a rate.
Problem Assume the growth of the population of Del Webb’s Sun City Hilton Head community was linear from 1996 to 2000, with a population of 198 in 1996 and a rate of growth of 705 people per year.
a. Write an equation for the population P of the community where x is the number of years after 1990.
Solution The population of 198 in 1996 corresponds to the point (6, 198) since the variable x corresponds to years after 1990. We’ll write the slope-intercept form of the line, P = mx + b, and substitute m into the equation. The rate of growth, 705 people per year, is the slope of the function. Therefore, the line describing the population is
P = 705x + b
To find the value of b, we need to substitute x = 6 and P = 198:
198 = 705(6) + b
-4032 = b
This gives us the equation,
P = 705x – 4032
b. Use the function to estimate the population in 2002.
Solution The year 2002 corresponds to x = 12. Substitute this value into the function to yield
P = 705(12) – 4032 = 4428
The population in 2002 will be 4428 people.
c. In what year will the population reach 10,000?
Solution In this part, set P = 10,000 and solve for x.
10000 = 705x – 4032
14032 = 705x
14032/705 = x
19.9 ≈ x
This corresponds to 1990 + 19.9 = 2009.9. So in the year 2009, the population will reach 10000. Since we are asking in what year, we DO NOT round up on the answer.