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Ch. 9 - Differential Equations
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 9, Problema 9.5.33

Solution of the logistic equation Use separation of variables to show that the solution of the initial value problem
P'(t) = rP (1-P/K), P(0) = P₀
is P(t) = K/((K/P₀ − 1)e⁻ʳᵗ + 1)

Guida verificata passo dopo passo
1
Start with the logistic differential equation given: \(P'(t) = rP \left(1 - \frac{P}{K}\right)\), where \(r\) and \(K\) are constants, and \(P(0) = P_0\) is the initial condition.
Rewrite the differential equation in separable form by dividing both sides by \(P \left(1 - \frac{P}{K}\right)\) and multiplying both sides by \(dt\): \(\frac{dP}{P \left(1 - \frac{P}{K}\right)} = r \, dt\).
Simplify the left side by expressing the denominator as a single fraction: \(P \left(1 - \frac{P}{K}\right) = P \left(\frac{K - P}{K}\right) = \frac{P(K - P)}{K}\), so the integral becomes \(\int \frac{K}{P(K - P)} \, dP = \int r \, dt\).
Use partial fraction decomposition to rewrite \(\frac{K}{P(K - P)}\) as \(\frac{A}{P} + \frac{B}{K - P}\), then solve for constants \(A\) and \(B\). After finding \(A\) and \(B\), integrate both sides: \(\int \left(\frac{A}{P} + \frac{B}{K - P}\right) dP = \int r \, dt\).
After integrating, apply the initial condition \(P(0) = P_0\) to solve for the constant of integration. Finally, solve the resulting equation for \(P(t)\) to obtain the explicit solution: \(P(t) = \frac{K}{\left(\frac{K}{P_0} - 1\right) e^{-rt} + 1}\).

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Logistic Differential Equation

The logistic differential equation models population growth with a carrying capacity, expressed as P'(t) = rP(1 - P/K). Here, r is the growth rate, K is the maximum population, and P(t) is the population at time t. It describes growth that slows as the population approaches K.
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Separation of Variables

Separation of variables is a method to solve differential equations by rewriting them so that each variable and its differential are on opposite sides. For the logistic equation, this involves isolating terms with P on one side and t on the other before integrating both sides.
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Separation of Variables

Initial Value Problem and Integration Constants

An initial value problem specifies the value of the solution at a particular point, here P(0) = P₀. After integrating, the constant of integration is determined using this initial condition to find the particular solution that fits the problem.
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Initial Value Problems