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Determine whether each differential equation is separable. (Do not solve it, just find whether it's separable.) $$ y^{\prime}=\ln (x+y) $$

Short Answer

Expert verified
The equation is not separable.

Step by step solution

01

Recognize the Form of the Equation

The given differential equation is \( y' = \ln(x + y) \). This is a first-order differential equation, where \( y' \) represents the derivative of \( y \) with respect to \( x \).
02

Concept of Separable Equations

A separable differential equation can be expressed in the form \( \frac{dy}{dx} = g(y) h(x) \), where the variables \( y \) and \( x \) can be separated on opposite sides of the equation.
03

Analyze the Given Equation

Examine the equation \( y' = \ln(x + y) \). To be separable, one should be able to rearrange it such that all \( y \)-terms are on one side, and all \( x \)-terms are on the other side. Currently, \( \ln(x + y) \) is intertwined with both \( x \) and \( y \).
04

Determine Separability

In the expression \( \ln(x + y) \), the variable \( x \) and \( y \) are additively combined under a logarithm function, which cannot be split into a product of functions of \( x \) and \( y \) independently. Therefore, it is not possible to separate the equation into \( g(y) h(x) \) form.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Separable Equations
Separable equations are a special type of differential equation where you can easily rearrange the terms to isolate variables on each side. Imagine you have an equation that ties two variables, like wiring a circuit board with clear lanes for each wire. If you can group all "\(y\)" terms on one side and all "\(x\)" terms on the other, you've got a separable equation. A classic example would be \(\frac{dy}{dx} = g(y)h(x)\). Here, the derivative \(\frac{dy}{dx}\) is expressed as a product where \(g(y)\) depends only on \(y\) and \(h(x)\) depends only on \(x\). This makes it a breeze to solve by integrating each side separately like taking a deep breath in, and out, without mixing them up.However, consider having a function where the variables are intertwined in a complex function like \(\ln(x + y)\). Unfortunately, here you're dealing with more of a tangled mess of lights than a straight path - hard to separate into a clean form. This means, in such cases, the equation isn't separable unless you can simplify it further by clever manipulation.
First-Order Differential Equations
First-order differential equations involve derivatives that are only first-degree, meaning the highest derivative you have is the first one, \(dy/dx\). Think of them like a snapshot in time, only focusing on the immediate change between variables. They often model real-world problems where the rate of change is crucial - like population growth, cooling of coffee, or speed of a falling object without air resistance.A typical form might be \(y' = f(x, y)\), where \(y'\) (or \(dy/dx\)) is given as a function of \(x\) and \(y\) themselves. They don’t wait on bigger changes or higher orders, but observe the instantaneous tilt or curve slope. When the gnome of the problem is quarreling about one variable at a time, these equations often end up being more straightforward to understand. However, when functions like \(\ln(x + y)\) tangle \(x\) and \(y\) together intricately, they might resist simple classification or solution methods, needing clever analysis or approximations.
Mathematical Analysis
Mathematical analysis is like the thoughtful detective of mathematics, seeking to understand and neatly untangle functions, limits, continuity, and differential equations. It becomes crucial when you want to rigorously ensure that solutions to problems are both valid and applicable in broader contexts. When delving into differential equations, analysis helps assess important properties, like existence and uniqueness of solutions. Can every initial condition bear a solution? Do small changes wildly affect outcomes, hinting at chaos or stability? Analysis provides the framework to dissect these possibilities. In the context of separable equations or first-order differentials, analysis helps determine if and how solutions behave over time - leading to insights about growth, decay, or oscillations. This is the soil from which deeper understandings about mathematical models grow, allowing us to predict and control systems accurately. Understanding intertwined functions requires looking beyond standard solutions to wrestle with more intricate structures that appear in regular problems.

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Most popular questions from this chapter

Solve each differential equation with the given initial condition. $$ \begin{array}{l} y^{\prime}+3 y=12 e^{x} \\ y(0)=5 \end{array} $$

A patient's ability to absorb a drug sometimes changes with time, and the dosage must therefore be adjusted. Suppose that the number of milligrams \(y(t)\) of a drug remaining in the patient's bloodstream after \(t\) hours satisfies $$ y^{\prime}=-\frac{1}{t} y+t \quad \text { (for } \left.t \geq 1\right) $$ $$ y(3)=5 $$ Solve this differential equation and initial condition to find the amount remaining in the bloodstream after \(t\) hours.

The following exercises require the use of a slope field program. For each differential equation: a. Use a graphing calculator slope field program to graph the slope field for the differential equation on the window [-5,5] by [-5,5]. b. Sketch the slope field on a piece of paper and draw a solution curve that follows the slopes and that passes through the given point. $$ \begin{array}{l} \frac{d y}{d x}=x \ln \left(y^{2}+1\right) \\ \text { point: }(0,-2) \end{array} $$

Explain why we cannot use an integrating factor \(I(x)\) to solve the nonlinear differential equation \(y^{\prime}+p(x) y^{2}=q(x) .\)

The following exercises require the use of a slope field program. For each differential equation and initial condition: a. Use a graphing calculator slope field program to graph the slope field for the differential equation on the window [-5,5] by [-5,5] b. Sketch the slope field on a piece of paper and draw a solution curve that follows the slopes and that passes through the point (0,2) c. Solve the differential equation and initial condition. d. Use your slope field program to graph the slope field and the solution that you found in part (c). How good was the sketch that you made in part (b) compared with the solution graphed in part (d)? $$ \left\\{\begin{array}{l} \frac{d y}{d x}=\frac{6 x^{2}}{y^{4}} \\ y(0)=2 \end{array}\right. $$

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