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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 9.4.37b

A bad loan Consider a loan repayment plan described by the initial value problem
B'(t)=0.03B−600,B(0)=40,000,
where the amount borrowed is B(0)=\$40,000, the monthly payments are \$600, and B(t) is the unpaid balance in the loan.
b. What is the most that you can borrow under the terms of this loan without going further into debt each month?

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Identify the differential equation given: \(B'(t) = 0.03B - 600\), where \(B(t)\) is the unpaid balance at time \(t\), and \(B(0) = 40,000\) is the initial loan amount.
Understand that the question asks for the maximum initial loan amount such that the balance does not increase over time, meaning the unpaid balance does not grow each month. This implies that the rate of change of the balance, \(B'(t)\), should be less than or equal to zero at the start (i.e., \(B'(0) \leq 0\)).
Set up the inequality using the differential equation at \(t=0\): \(B'(0) = 0.03 B(0) - 600 \leq 0\). This inequality ensures the loan balance does not increase initially.
Solve the inequality for \(B(0)\): \(0.03 B(0) \leq 600\), which leads to \(B(0) \leq \frac{600}{0.03}\). This gives the maximum loan amount that can be borrowed without the balance increasing.
Interpret the result: the maximum loan amount is the value of \(B(0)\) found above, which ensures the loan balance will not grow over time under the given repayment plan.

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주요 개념

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

Initial Value Problem (IVP)

An initial value problem involves a differential equation along with a specified value of the unknown function at a starting point. Here, B'(t) = 0.03B - 600 with B(0) = 40,000 means the rate of change of the loan balance depends on the current balance and payments, starting from $40,000 owed.
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가이드 코스
05:03
Initial Value Problems

Solving First-Order Linear Differential Equations

This type of differential equation can be solved using integrating factors or separation of variables. It helps find the function B(t) describing the loan balance over time, which is essential to analyze how the balance changes with interest and payments.
추천 영상:
06:06
Solving Separable Differential Equations

Equilibrium or Steady-State Solution

The steady-state solution occurs when the loan balance stops changing, i.e., B'(t) = 0. Finding this equilibrium helps determine the maximum loan amount that can be maintained without increasing debt, by setting the growth from interest equal to the monthly payment.
추천 영상:
04:00
Solutions to Basic Differential Equations
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교과서 질문

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} Drug infusion The delivery of a drug (such as an antibiotic) through an intravenous line may be modeled by the differential equation m'(t) + km(t) = I, where m(t) is the mass of the drug in the blood at time t ≥ 0, k is a constant that describes the rate at which the drug is absorbed, and I is the infusion rate.


b. Graph the solution for I = 10 mg/hr and k = 0.05 hr⁻¹.

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교과서 질문

{Use of Tech} Chemical rate equations Let y(t) be t he concentration of a substance in a chemical reaction (typical units are moles/liter). The change in the concentration, under appropriate conditions, is modeled by the equation dy/dt=-ky^n for t≥0, where k>0 is a rate constant and the positive integer n is the order of the reaction.

b. Solve the initial value problem for a second-order reaction (n=2) assuming y(0)=y0.

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교과서 질문

Convergence of Euler's method Suppose Euler's method is applied to the initial value problem y′(t) = ay, y(0) = 1, which has the exact solution y(t) = eᵃᵗ. For this exercise, let h denote the time step (rather than Δt). The grid points are then given by tₖ = kh. We let uₖ be the Euler approximation to the exact solution y(tₖ), for k = 0, 1, 2, ...

b. Show by substitution that uₖ = (1 + ah)ᵏ is a solution of the equations in part (a), for k = 0, 1, 2, ...

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교과서 질문

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 − xy, y′(t) = −y + xy

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교과서 질문

29–32. {Use of Tech} Errors in Euler’s method Consider the following initial value problems.


b. Using the exact solution given, compute the errors in the Euler approximations at t=0.2 and t=0.4.


y′(t) = 2t + 1, y(0) = 0; y(t) = t² + t

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교과서 질문

brOrthogonal trajectories Two curves are orthogonal to each other if their tangent lines are perpendicular at each point of intersection. A family of curves forms orthogonal trajectories with another family of curves if each curve in one family is orthogonal to each curve in the other family. Use the following steps to find the orthogonal trajectories of the family of ellipses 2x² + y² = a²


b. The family of trajectories orthogonal to 2x² + y² = a² satisfies the differential equation dy/dx = y/(2x). Why?

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