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Ch. 9 - Differential Equations
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243당신이 사용하는 게 아니라요?교과서 변경
9장, 문제 9.5.28b

27–30. Predator-prey models Consider the following pairs of differential equations that model a predator-prey system with populations x and y. In each case, carry out the following steps.
b. Find the lines along which x'(t) = 0. Find the lines along which y'(t) = 0.


x′(t) = 2x − 4xy, y′(t) = −y + 2xy

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Identify the given system of differential equations: \(x'(t) = 2x - 4xy\) and \(y'(t) = -y + 2xy\).
To find the lines where \(x'(t) = 0\), set the right-hand side of the first equation equal to zero: \(2x - 4xy = 0\).
Factor the equation \(2x - 4xy = 0\) to get \(2x(1 - 2y) = 0\). This implies either \(x = 0\) or \(1 - 2y = 0\).
Solve \(1 - 2y = 0\) to find \(y = \frac{1}{2}\). So, the lines where \(x'(t) = 0\) are \(x = 0\) and \(y = \frac{1}{2}\).
Next, find the lines where \(y'(t) = 0\) by setting \(-y + 2xy = 0\). Factor to get \(y(-1 + 2x) = 0\), which implies \(y = 0\) or \(x = \frac{1}{2}\).

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Predator-Prey Model

A predator-prey model describes the interaction between two species: one as prey (x) and the other as predator (y). The populations change over time based on birth, death, and interaction rates, often modeled by coupled differential equations. Understanding these dynamics helps analyze population stability and oscillations.
추천 영상:
09:29
Exponential Growth & Decay

Nullclines (Lines where derivatives are zero)

Nullclines are curves in the phase plane where the rate of change of one variable is zero (x' = 0 or y' = 0). They help identify equilibrium points and the system's behavior by showing where populations neither increase nor decrease. Finding nullclines is essential for analyzing system stability.
추천 영상:
가이드 코스
05:13
Slopes of Tangent Lines

Solving for Equilibrium Conditions

Equilibrium points occur where both derivatives are zero simultaneously (x' = 0 and y' = 0). Solving these conditions involves setting the differential equations to zero and finding the corresponding population values. These points indicate steady states of the system and are key to understanding long-term behavior.
추천 영상:
06:06
Solving Separable Differential Equations
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33–36. {Use of Tech} Computing Euler approximations Use a calculator or computer program to carry out the following steps.

b. Using the exact solution (also given), find the error in the approximation to y(T) (only at the right endpoint of the time interval).


y′(t) = -2y, y(0) = 1; Δt = 0.2, T = 2; y(t) = e⁻²ᵗ

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Explain why or why not Determine whether the following statements are true and give an explanation or counterexample.

b. The general solution of the separable equation y'(t) = t/(y' + 10y⁴) can be expressed explicitly with y in terms of t.

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52-56. In this section, several models are presented and the solution of the associated differential equation is given. Later in the chapter, we present methods for solving these differential equations.


{Use of Tech} Tumor growth The growth of cancer tumors may be modeled by the Gompertz growth equation. Let M(t) be the mass of a tumor, for t ≥ 0. The relevant initial value problem is:


dM/dt = -rM(t)ln(M(t)/K), M(0) = M₀,


where r and K are positive constants and 0 < M₀ < K.


b. Graph the solution for M₀ = 100 and r = 0.05.

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23–26. Stirred tank reactions For each of the following stirred tank reactions, carry out the following analysis.

b. Solve the initial value problem.


A 500-L tank is initially filled with pure water. A copper sulfate solution with a concentration of 20 g/L flows into the tank at a rate of 4 L/min. The thoroughly mixed solution is drained from the tank at a rate of 4 L/min.

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17–20. Increasing and decreasing solutions Consider the following differential equations. A detailed direction field is not needed.


b. In what regions are solutions increasing? Decreasing?


y'(t) = (y−1)(1+y)

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Properties of stirred tank solutions


b. Verify that M(0) = M₀

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