Mathematics

Problem

The population density

P

(in persons per square kilometer) of a circular city depends on its area

A

(in square kilometers) according to

P(A)=\frac{5000}{1+0.001A}.

The area satisfies

A=\pi r^{2}

, where the city’s radius

r

(in kilometers) changes with time following

r(t)=0.5t^{2}+2t

, with

t

in hours. Find

\dfrac{dP}{dt}

t=3

hours. Which value is closest to your result? a)

-5.6\times10^{2}\;\text{persons/km}^{2}\!/\text{h}

9.1\times10^{2}\;\text{persons/km}^{2}\!/\text{h}

-9.1\times10^{2}\;\text{persons/km}^{2}\!/\text{h}

-1.2\times10^{3}\;\text{persons/km}^{2}\!/\text{h}