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PreCalc Question

  • Thread starter GreenPrint
  • Start date
  • #1
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Homework Statement



The average growth rate of the population of a certain city is 7.5% per year. The city's population is now 22,750 people. What is the expected population in 10 years?

Homework Equations



I was always taught this formula for exponential growth

N(t) = N_0 e^(kt)
N = population
N_0 = population at t(0)
e = 2.7...
k = some positive constant
t = time

Here's what my teacher wrote on my paper for the formula

f(x) = C(1 + r)^x
f(x) = population
C = initial population
r = growth rate

The Attempt at a Solution



no i dont understand how to do this exactly because I don't know what to use for the constant k

so i used the second one

22750 (1 + .075)^10 = 46888.4680

now what I don't udnerstand is that this really makes no sense at all becasue if I wanted to fidn the population at 10 minutes or ten centuries and just plugged in 10 into the equation with no units at all I would get the same exact answer. Can youp please tell me how to go about reasoning this out... THANKS!
 

Answers and Replies

  • #2
Char. Limit
Gold Member
1,204
13
The reason it works is because the growth rate must be defined using the inverse unit of whichever unit of time you're using.

In this case, your growth rate is defined at 7.5% per year. If you used centuries for t, you'd have to find the growth rate per century.
 

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