Math Problem Statement

Find the the solution of the differential equations... xy' + y ln(x) = y ln(y) + y

Solution

The given differential equation is:

xdydx+yln(x)=yln(y)+yx \frac{dy}{dx} + y \ln(x) = y \ln(y) + y

Step 1: Simplify the equation

First, divide the entire equation by yy (assuming y0y \neq 0):

xydydx+ln(x)=ln(y)+1\frac{x}{y} \frac{dy}{dx} + \ln(x) = \ln(y) + 1

Step 2: Separate the variables

Rearrange the terms to separate the variables:

xydydx=ln(y)ln(x)+1\frac{x}{y} \frac{dy}{dx} = \ln(y) - \ln(x) + 1

Recognize that ln(y)ln(x)=ln(yx)\ln(y) - \ln(x) = \ln\left(\frac{y}{x}\right), so the equation becomes:

xydydx=ln(yx)+1\frac{x}{y} \frac{dy}{dx} = \ln\left(\frac{y}{x}\right) + 1

Step 3: Change of variables

Let's introduce a substitution to simplify the equation. Define:

v=yxv = \frac{y}{x}

Then, y=vxy = vx, and differentiating with respect to xx:

dydx=v+xdvdx\frac{dy}{dx} = v + x \frac{dv}{dx}

Substitute into the equation:

xvx(v+xdvdx)=ln(v)+1\frac{x}{vx} \left(v + x \frac{dv}{dx}\right) = \ln(v) + 1

Simplify:

1v(v+xdvdx)=ln(v)+1\frac{1}{v}(v + x \frac{dv}{dx}) = \ln(v) + 1

1+xvdvdx=ln(v)+11 + \frac{x}{v} \frac{dv}{dx} = \ln(v) + 1

Subtract 1 from both sides:

xvdvdx=ln(v)\frac{x}{v} \frac{dv}{dx} = \ln(v)

Step 4: Separate and integrate

Separate the variables:

1ln(v)dv=1xdx\frac{1}{\ln(v)} dv = \frac{1}{x} dx

Now, integrate both sides:

1ln(v)dv=1xdx\int \frac{1}{\ln(v)} dv = \int \frac{1}{x} dx

The integral on the right-hand side is straightforward:

1xdx=lnx+C1\int \frac{1}{x} dx = \ln|x| + C_1

The integral on the left-hand side is a special function, known as the logarithmic integral, but in a simpler form it can be solved as:

Li(v)=lnx+C\text{Li}(v) = \ln|x| + C

Final Answer:

The general solution of the differential equation is: In implicit form using special functions.

Would you like more details on this, or other questions:

  1. How to calculate special functions?
  2. Alternative methods for solution
  3. Particular conditions
  4. Functions relations with integral result.

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Math Problem Analysis

Mathematical Concepts

Differential Equations
Change of Variables
Logarithmic Functions
Separation of Variables

Formulas

x dy/dx + y ln(x) = y ln(y) + y
v = y/x
dy/dx = v + x dv/dx

Theorems

Separation of Variables
Logarithmic Integral

Suitable Grade Level

Undergraduate Level