Math Problem Statement

Solve the initial value problem (1-3t)dy/dt-y = 0, y(2) = -6

Solution

To solve the given initial value problem:

(13t)dydty=0,y(2)=6(1 - 3t) \frac{dy}{dt} - y = 0, \quad y(2) = -6

we proceed step by step.

Step 1: Rearranging the Equation

We can rewrite the equation in a form that is easier to handle. Begin by isolating the derivative dydt\frac{dy}{dt}:

(13t)dydt=y(1 - 3t) \frac{dy}{dt} = y

dydt=y13t\frac{dy}{dt} = \frac{y}{1 - 3t}

This is a separable differential equation, so we can separate the variables yy and tt.

Step 2: Separating Variables

We rearrange the equation to separate the variables:

dyy=dt13t\frac{dy}{y} = \frac{dt}{1 - 3t}

Step 3: Integrating Both Sides

Now, we integrate both sides:

1ydy=113tdt\int \frac{1}{y} \, dy = \int \frac{1}{1 - 3t} \, dt

The integral on the left-hand side is straightforward:

lny=113tdt\ln|y| = \int \frac{1}{1 - 3t} \, dt

For the integral on the right-hand side, use substitution. Let u=13tu = 1 - 3t, so du=3dtdu = -3dt, and dt=du3dt = -\frac{du}{3}.

113tdt=1u(du3)=13lnu+C=13ln13t+C\int \frac{1}{1 - 3t} \, dt = \int \frac{1}{u} \left(-\frac{du}{3}\right) = -\frac{1}{3} \ln|u| + C = -\frac{1}{3} \ln|1 - 3t| + C

So we have:

lny=13ln13t+C\ln|y| = -\frac{1}{3} \ln|1 - 3t| + C

Step 4: Solving for yy

Now exponentiate both sides to eliminate the natural logarithm:

y=e13ln13t+C|y| = e^{-\frac{1}{3} \ln|1 - 3t| + C}

This simplifies to:

y=eC13t13|y| = e^C \cdot |1 - 3t|^{-\frac{1}{3}}

Let A=eCA = e^C, where AA is a constant, so:

y=A13t13|y| = A \cdot |1 - 3t|^{-\frac{1}{3}}

Thus, the general solution is:

y=A(13t)13y = A \cdot (1 - 3t)^{-\frac{1}{3}}

Step 5: Using the Initial Condition

We are given the initial condition y(2)=6y(2) = -6. Substitute t=2t = 2 and y=6y = -6 into the general solution:

6=A(13(2))13=A(5)13-6 = A \cdot (1 - 3(2))^{-\frac{1}{3}} = A \cdot (-5)^{-\frac{1}{3}}

Since (5)13=151/3(-5)^{-\frac{1}{3}} = -\frac{1}{5^{1/3}}, we get:

6=A(151/3)-6 = A \cdot \left(-\frac{1}{5^{1/3}}\right)

Solving for AA:

A=651/3A = 6 \cdot 5^{1/3}

Thus, the particular solution is:

y=651/3(13t)13y = 6 \cdot 5^{1/3} \cdot (1 - 3t)^{-\frac{1}{3}}

Final Answer:

The solution to the initial value problem is:

y=651/3(13t)13y = 6 \cdot 5^{1/3} \cdot (1 - 3t)^{-\frac{1}{3}}


Would you like further clarification on any part of the solution?

Here are some related questions you could explore:

  1. How does the solution change if the differential equation is altered to a non-separable form?
  2. What happens if the initial condition is different, say y(1)=3y(1) = 3?
  3. How do we solve a first-order linear differential equation using an integrating factor?
  4. How does the behavior of the solution change as tt approaches 13\frac{1}{3}?
  5. What would the solution look like if the coefficient of dydt\frac{dy}{dt} were not linear?

Tip: When solving separable differential equations, always be careful about domain restrictions, especially if the function inside the logarithm or a power becomes zero!

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Math Problem Analysis

Mathematical Concepts

Differential Equations
Separable Equations
Initial Value Problems
Integration

Formulas

dy/dt = y / (1 - 3t)
ln|y| = -1/3 ln|1 - 3t| + C
y = A * (1 - 3t)^(-1/3)

Theorems

Separation of Variables
Integration of Rational Functions

Suitable Grade Level

Grades 11-12