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Question

A city's population is 1000 and growing at 5%5\% a year.Find a formula for the population at time tt years from now assuming that the 5%5\% per year is an:
(i) Annual rate (ii) Continuous annual rate

Solution

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(i)\pmb{(i)}

To find the formula for the population at time tt years from now we will use the information that is given in the text of the exercise. We now that:

P(0)=P0=1000r=5%=0.05a=1+r=1.05\begin{align*} P(0)&=P_0=1000\tag{\footnotesize\textcolor{#4257b2}{initial population}}\\ r&=5\%=0.05\tag{\footnotesize\textcolor{#4257b2}{annual growth rate}}\\ & \Rightarrow a=1+r=1.05 \end{align*}

We see that the formula for the city's population in this case is:

P(t)=1000(1.05)t\boxed{P(t)=1000\cdot (1.05)^t}

(ii)\pmb{(ii)}

In this case (continuous annual rate) we have that:

P0=1000k=5%=0.05\begin{align*} P_0&=1000\\ k&=5\%=0.05 \end{align*}

so we have that the formula for the city's population is given by:

P(t)=1000e0.05t\boxed{ P(t)=1000\cdot e^{0.05\cdot t} }

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