로지스틱 방정식

(a) Let $y$ be the per capita growth rate as a function of $b$. Using the given data in the problem statement, we know that this linear function passes through the points $(6000,3)$ and $(9000,2)$. The equation of this line is thus $$y - 3 = -\frac{1}{3000}(b - 6000)$$ so $y = (3 + 2) - \frac{1}{3000}b = 5 - \frac{1}{3000}b$. Since the logistic equation comes from taking the product of the population at time $t$ with its per capita growth rate at that time, we find that $$\frac{db}{dt} = b \left( 5 - \frac{1}{3000}b \right) = \frac{1}{3000} b(15000 - b)$$ (b) From the form of the logistic equation, we see that the carrying capacity for the bacteria is $b = 15000$, which is a stable equilibrium for the population. (c) The bacteria is increasing most rapidly when $\frac{db}{dt}$ is greatest. Since $$\frac{db}{dt} = \frac{1}{3000} b(15000 - b)$$ is a quadratic function of $b$, $\frac{db}{dt}$ is maximized at the vertex of this concave down parabola, which occurs halfway between its zeros: $b = 0$ and $b = 15000$. Thus, when $b = 7500$, the population is growing fastest. To find the exact $t$-value when this occurs, we'd need to know an initial condition like in the next part of the problem. (d) Using $b(0) = 1000$, we solve the logistic equation IVP with $$\frac{db}{dt} = \frac{1}{3000} b(15000 - b)\,.$$ Using $N = 15000$, $k = \frac{1}{3000}$, and $b_0 = 1000$, we know from the standard form of the solution to the logistic equation that $$b(t) = \frac{15000}{\left(\frac{15000-1000}{1000}\right) e^{-5t} + 1} = \frac{15000}{14e^{-5t} + 1}\,.$$ To find how long it takes the bacteria to reach 80% of the carrying capacity, we solve the equation $$0.8 \cdot 15 = \frac{15000}{14e^{-5t} + 1}\,.$$ That equation implies that $$0.8 = \frac{1}{14e^{-5t} + 1}\,,$$ so $14e^{-5t} + 1 = 1.2$ and thus $e^{-5t} = 0.2/14 = 1/70$. It follows that $t = -\frac{1}{5} \ln(1/70) \approx 0.8497$ days.