Question 23 of 96intermediate🔧 ApplyLong Answer4 marks

For each of the differential equations in Exercises 11 to 14, find a particular solution satisfying the given condition:

x(x21)dydx=1;y=0 when x=2x (x^2 - 1) \frac{dy}{dx} = 1; \quad y = 0 \text{ when } x = 2

Correct Answer

y=ln(2x213x)y = \ln\left( \frac{2\sqrt{x^2-1}}{\sqrt{3}x} \right)

Exercise: EXERCISE 9.3 | Q: 12 | (Chapter: 12)
For More Understanding

Explanation

The differential equation is solved using the method of separation of variables. The expression is rearranged to separate terms with yy and terms with xx on opposite sides.

The integration on the right side requires partial fraction decomposition to handle the term 1x(x21)\frac{1}{x(x^2-1)}.

After integrating and finding the general solution, the given condition y=0y=0 when x=2x=2 is substituted to find the value of the constant of integration, yielding the particular solution.

Solution Steps

  1. Step 1: Separate the variables.

    The given differential equation is:

    x(x21)dydx=1x(x^2-1)\frac{dy}{dx}=1

    Rewriting this, we get:

    dy=dxx(x21)dy = \frac{dx}{x(x^2-1)}

  2. Step 2: Integrate both sides.

    Integrating the LHS gives yy. For the RHS, use partial fractions for 1x(x21)\frac{1}{x(x^2-1)}.

    1x(x21)=1x(x1)(x+1)=1x+12(x1)+12(x+1)\frac{1}{x(x^2-1)} = \frac{1}{x(x-1)(x+1)} = \frac{-1}{x} + \frac{1}{2(x-1)} + \frac{1}{2(x+1)}

    Now, integrate:

    dy=(1x+12(x1)+12(x+1))dx\int dy = \int \left( \frac{-1}{x} + \frac{1}{2(x-1)} + \frac{1}{2(x+1)} \right) dx

  3. Step 3: Find the general solution.

    y=lnx+12lnx1+12lnx+1+Cy = -\ln|x| + \frac{1}{2}\ln|x-1| + \frac{1}{2}\ln|x+1| + C

    Combining the logarithmic terms:

    y=12[lnx1+lnx+1]lnx+Cy = \frac{1}{2}[\ln|x-1| + \ln|x+1|] - \ln|x| + C

    y=12lnx21lnx+Cy = \frac{1}{2}\ln|x^2-1| - \ln|x| + C

    y=ln(x21x)+Cy = \ln\left( \frac{\sqrt{x^2-1}}{|x|} \right) + C

  4. Step 4: Apply the given condition to find C.

    Given condition: y=0y=0 when x=2x=2.

    Substituting these values:

    0=ln(2212)+C0 = \ln\left( \frac{\sqrt{2^2-1}}{2} \right) + C

    0=ln(32)+C0 = \ln\left( \frac{\sqrt{3}}{2} \right) + C

    C=ln(32)=ln(23)C = -\ln\left( \frac{\sqrt{3}}{2} \right) = \ln\left( \frac{2}{\sqrt{3}} \right)

  5. Step 5: Write the particular solution.

    Substitute CC back into the general solution:

    y=ln(x21x)+ln(23)y = \ln\left( \frac{\sqrt{x^2-1}}{x} \right) + \ln\left( \frac{2}{\sqrt{3}} \right)

    y=ln(2x213x)y = \ln\left( \frac{2\sqrt{x^2-1}}{\sqrt{3}x} \right)

    y=ln(2x213x)y = \ln\left( \dfrac{2\sqrt{x^2-1}}{\sqrt{3}\,x} \right)