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Solve the initial -value problem. $$ \left(1+e^{x}\right) \frac{d y}{d x}+e^{x} y=\sin x, \quad y(0)=\frac{1}{2} $$

Short Answer

Expert verified
The solution to the initial-value problem is: \(y(x) = e^{-e^x} \left( \int e^{e^x} \sin x dx + \frac{1}{2} \right)\) Note that it is not possible to find an explicit formula for the integral \(\int e^{e^x} \sin x dx\). However, numerical integration techniques can be used to find an approximation or a numerical solution if required.

Step by step solution

01

Identify the integrating factor

The given ODE is a linear ODE of the form \(\frac{dy}{dx} + P(x) y = Q(x)\), where \(P(x) = e^x\) and \(Q(x) = \frac{\sin x}{1+e^x}\). To solve this, we will first find the integrating factor. The integrating factor is given by the function \(\mu(x)\) = \(e^{\int P(x) dx}\). In this case, we have: \[ \mu(x) = e^{\int e^x dx} = e^{e^x} \]
02

Multiply both sides of the ODE by the integrating factor

Now, we will multiply both sides of the ODE by the integrating factor \(\mu(x) = e^{e^x}\): \[ e^{e^x}\left(1+e^{x}\right) \frac{dy}{dx}+ e^{e^x} e^{x} y= e^{e^x} \sin x \]
03

Integrate the resulting expression with respect to x

The left side of the equation is the derivative of the product of the integrating factor and y(x) with respect to x. So, we have: \[ \frac{d}{dx}(e^{e^x} y) = e^{e^x} \sin x \] Now, we will integrate both sides of the equation with respect to x: \[ \int \frac{d}{dx}(e^{e^x} y) dx = \int e^{e^x} \sin x dx \] Integrating both sides: \[ e^{e^x} y = \int e^{e^x} \sin x dx + C \]
04

Solve for y(x) and use the initial value y(0) to find the constant of integration

Now, we will solve for y(x): \[ y(x) = e^{-e^x} \left( \int e^{e^x} \sin x dx + C \right) \] Applying the initial condition y(0) = 1/2: \[ \frac{1}{2} = e^{-e^0} \left( \int_0^0 e^{e^x} \sin x dx + C \right) \] Since the integral from 0 to 0 is zero, we have: \[ \frac{1}{2} = e^0 ( 0 + C ) \] Thus, C = 1/2. Therefore, the solution to the initial-value problem is: \[ y(x) = e^{-e^x} \left( \int e^{e^x} \sin x dx + \frac{1}{2} \right) \] Note that, in general, finding an explicit formula for the integral \(\int e^{e^x} \sin x dx\) is not possible. However, if we needed a numerical solution or an approximation, we could use numerical integration techniques.

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Key Concepts

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

Integrating Factor
An integrating factor is a mathematical tool used to solve linear ordinary differential equations (ODEs). It is essentially a function, typically denoted by \( \mu(x) \) that, when multiplied by the ODE, renders the equation integrable, allowing for a clear path to the solution.

The integrating factor is computed as \( e^{\int P(x) dx} \) where \( P(x) \) is the coefficient of \( y \) in the linear ODE. Once we have the integrating factor, multiplying it across the ODE simplifies the problem substantially, turning the left-hand side into a product rule derivative. This transformation is the key that unravels the ODE, setting up for easy integration.
Linear Ordinary Differential Equation
A linear ordinary differential equation is an equation that involves an unknown function and its derivatives. It follows the general form \( \frac{dy}{dx} + P(x)y = Q(x) \) for first-order linear ODEs. The solutions to these equations are critical in math and engineering because they model various physical phenomena.

To solve linear ODEs, certain techniques like finding an integrating factor come in handy. The given problem presents a first-order linear ODE where the integrating factor method is used. Once the integrating factor is determined and the equation is multiplied by it, the left side usually transforms into the derivative of a product, paving the way for direct integration.
Integration
Integration is a fundamental operation in calculus, the inverse of differentiation, and is used to find quantities like areas, volumes, and the total accumulation of a quantity. When dealing with differential equations, after multiplying by the integrating factor, we perform integration on both sides to move closer to finding the solution of the unknown function \( y(x) \).

In our initial-value problem, after applying the integrating factor, integration becomes the primary tool to get us to a general solution. The result of this integration typically includes an arbitrary constant. When given a specific initial condition, such as \( y(0) \) in our case, this constant can be determined, thus providing a particular solution to the ODE.
Exponential Functions
Exponential functions are pivotal in solving certain ODEs, especially when combined with integrating factors. These functions are of the form \( e^{x} \), where \( e \) is the base of the natural logarithm, and the variable \( x \) is the exponent. The unique property of the exponential function that \( \frac{d}{dx}e^{x} = e^{x} \) makes it deeply connected to growth and decay processes in natural systems.

In the context of our problem, the integrating factor itself is an exponential function that contains another exponential function, \( e^{e^x} \), which dramatically simplifies the process upon multiplication. However, integrating such compounded exponential functions, as in \( \int e^{e^x} \sin x dx \), could be complex or even unsolvable with elementary functions, requiring numerical methods for approximation.

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