Mathematics

Mathematics

Problem

A fish population PP (in hundreds) in a secluded lake satisfies dPdt=0.08 P (500−P)\dfrac{dP}{dt}=0.08\,P\,(500-P), where tt is measured in months. At what population size is the instantaneous growth rate dPdt\dfrac{dP}{dt} greatest? a) 333333 b) 500500 c) 250250 d) 125125
Intuitive explanation

This is a logistic growth problem where you need to find the maximum growth rate, not the maximum population. The key insight is that dPdt\frac{dP}{dt} itself is a function of PP—treat it as a single-variable function and find where its derivative equals zero. Since dPdt=0.08P(500−P)\frac{dP}{dt} = 0.08P(500-P) is a parabola in PP, its peak occurs at the midpoint between its roots.

Intuitive explanation

This is a logistic growth problem where you need to find the maximum growth rate, not the maximum population. The key insight is that dPdt\frac{dP}{dt} itself is a function of PP—treat it as a single-variable function and find where its derivative equals zero. Since dPdt=0.08P(500−P)\frac{dP}{dt} = 0.08P(500-P) is a parabola in PP, its peak occurs at the midpoint between its roots.

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