a citys population was 10000 in 1980 and 13000 in 1990 assum
a city\'s population was 10,000 in 1980 and 13,000 in 1990. assuming an exponential growth rate of (dy/dt=ky), to the nearest thousand, estimate the cities population in 2000.
Solution
We have
dy/dt=ky ==> dy/y=kdt
Integrate both sides with respect to t to get
lny=kt+c1 ==> y=Cekt
Now, use the given data to solve for k. Let t=# of years after 1980. Then at t=0, y=10,000 and clearly C=10,000. At t=10, y=13,000, so we can use this to solve for k. We get
13,000=10,000e10k ==> 13/10=e10k ==> ln(13/10)=10k ==> ln(13/10)/10=k.026236. Now, we see that the exact growth equation is
y=10,000e.026236t
Plug in t=20 (20 years after 1980) to estimate the population in 2000. We get
y=10,000e.026236*20=16,900
