Math Problem Statement

Find the solution to the differential equation dy/dx = (5xy) / (ln(y))^2 which passes through the point (0, e). Express your answer as ln y = ?

Solution

Let's solve the given differential equation step by step.

Given Differential Equation:

dydx=5xy(lny)2\frac{dy}{dx} = \frac{5xy}{(\ln y)^2} We are also given the initial condition that the solution passes through the point (0,e)(0, e).

Step 1: Separate the Variables

To solve this differential equation, it's beneficial to separate the variables yy and xx. Let's rewrite the equation in a form that allows us to integrate:

1y(lny)2dy=5xdx\frac{1}{y} (\ln y)^2 \, dy = 5x \, dx

Step 2: Integrate Both Sides

Now, integrate both sides with respect to their respective variables.

  1. Left-hand side:

(lny)2ydy\int \frac{(\ln y)^2}{y} \, dy

Let's perform a substitution to make this integration easier. Set:

u=lnydudy=1ydy=yduu = \ln y \quad \Rightarrow \quad \frac{du}{dy} = \frac{1}{y} \quad \Rightarrow \quad dy = y \, du

Substituting this in, we get:

(lny)2ydy=u2du=u33+C=(lny)33+C\int \frac{(\ln y)^2}{y} \, dy = \int u^2 \, du = \frac{u^3}{3} + C = \frac{(\ln y)^3}{3} + C

  1. Right-hand side:

5xdx=5x22+C1\int 5x \, dx = \frac{5x^2}{2} + C_1

Step 3: Combine the Integrals

Combining the results from both sides, we have:

(lny)33=5x22+C\frac{(\ln y)^3}{3} = \frac{5x^2}{2} + C

Step 4: Solve for the Constant CC

We are given the point (0,e)(0, e), which we will use to determine the constant CC.

Substituting x=0x = 0 and y=ey = e:

(lne)33=5(0)22+C\frac{(\ln e)^3}{3} = \frac{5(0)^2}{2} + C

Since lne=1\ln e = 1:

133=CC=13\frac{1^3}{3} = C \quad \Rightarrow \quad C = \frac{1}{3}

Step 5: Write the General Solution

Now substitute back the value of CC:

(lny)33=5x22+13\frac{(\ln y)^3}{3} = \frac{5x^2}{2} + \frac{1}{3}

To solve for lny\ln y:

(lny)3=3(5x22+13)(\ln y)^3 = 3 \left( \frac{5x^2}{2} + \frac{1}{3} \right)

(lny)3=15x22+1(\ln y)^3 = \frac{15x^2}{2} + 1

Taking the cube root on both sides:

lny=(15x22+1)1/3\ln y = \left( \frac{15x^2}{2} + 1 \right)^{1/3}

Final Answer

lny=(15x22+1)1/3\ln y = \left( \frac{15x^2}{2} + 1 \right)^{1/3}

Would you like more details on any part of this solution?

Here are some follow-up questions you might be interested in:

  1. How can we verify if a solution satisfies a differential equation?
  2. What are some other methods to solve differential equations besides separation of variables?
  3. How does the initial condition affect the uniqueness of a solution in differential equations?
  4. What are the possible applications of such differential equations in physics or engineering?
  5. How would the solution change if we had a different initial point?

Tip: When dealing with logarithmic differential equations, substitution methods (like u=lnyu = \ln y) can greatly simplify the integration process.

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Math Problem Analysis

Mathematical Concepts

Differential Equations
Separation of Variables
Initial Conditions

Formulas

dy/dx = (5xy) / (ln y)^2
Separation of Variables: (1/y) * (ln y)^2 dy = 5x dx
Integral of (ln y)^2 / y dy
Substitution u = ln y

Theorems

Separation of Variables
Substitution Method in Integration

Suitable Grade Level

Undergraduate level (Calculus/Differential Equations)