Question 69 of 96intermediate🔧 ApplyLong Answer4 marks

For each of the differential equations given in Exercises 1 to 12, find the general solution:

xdydx+2y=x2log⁡xx \frac{dy}{dx} + 2y = x^2 \log x
Correct Answer

The given differential equation is xdydx+2y=x2log⁡xx \frac{dy}{dx} + 2y = x^2 \log x. To solve, we first rewrite it in the standard linear form dydx+P(x)y=Q(x)\frac{dy}{dx} + P(x) y = Q(x). Dividing both sides by xx (x≠0x \neq 0), we get:

dydx+2xy=xlog⁡x\frac{dy}{dx} + \frac{2}{x} y = x \log x

Here, P=2xP = \frac{2}{x} and Q=xlog⁡xQ = x \log x.

The integrating factor (I.F.) is:

e∫P dx=e∫2x dx=e2log⁡x=x2e^{\int P \, dx} = e^{\int \frac{2}{x} \, dx} = e^{2 \log x} = x^2

Multiplying both sides of the linear equation by the I.F., we obtain:

x2dydx+2x y=x3log⁡xx^2 \frac{dy}{dx} + 2x \, y = x^3 \log x

The left-hand side is the derivative of yx2y x^2 with respect to xx, i.e.,

ddx(yx2)=x3log⁡x\frac{d}{dx} (y x^2) = x^3 \log x

Integrating both sides with respect to xx, we get:

yx2=∫x3log⁡x dx+Cy x^2 = \int x^3 \log x \, dx + C

To evaluate the integral, we use integration by parts. Let u=log⁡xu = \log x and dv=x3 dxdv = x^3 \, dx. Then du=1xdxdu = \frac{1}{x} dx and v=x44v = \frac{x^4}{4}. Thus,

∫x3log⁡x dx=x44log⁡x−∫x44⋅1x dx=x44log⁡x−14∫x3 dx=x44log⁡x−x416\int x^3 \log x \, dx = \frac{x^4}{4} \log x - \int \frac{x^4}{4} \cdot \frac{1}{x} \, dx = \frac{x^4}{4} \log x - \frac{1}{4} \int x^3 \, dx = \frac{x^4}{4} \log x - \frac{x^4}{16}

Substituting back, we have:

yx2=x44log⁡x−x416+Cy x^2 = \frac{x^4}{4} \log x - \frac{x^4}{16} + C

Finally, dividing by x2x^2 yields the general solution:

y=x24log⁡x−x216+Cx2y = \frac{x^2}{4} \log x - \frac{x^2}{16} + \frac{C}{x^2}
Exercise: EXERCISE 9.5 | Q: 6 | (Page 29)
For More Understanding

Explanation

This question is from Exercise 9.5, which deals with linear differential equations. The solution method follows Example 16 in the chapter, where the equation is put into standard form and an integrating factor is used. The integration step requires integration by parts, a standard technique taught earlier.

Solution Steps

  1. Step 1: Rewrite the equation in standard linear form:

    dydx+2xy=xlog⁡x\frac{dy}{dx} + \frac{2}{x} y = x \log x
  2. Step 2: Identify P=2xP = \frac{2}{x}, Q=xlog⁡xQ = x \log x.

  3. Step 3: Compute integrating factor:

    I.F.=e∫P dx=e∫2x dx=x2\text{I.F.} = e^{\int P \, dx} = e^{\int \frac{2}{x} \, dx} = x^2
  4. Step 4: Multiply both sides by I.F.:

    x2dydx+2x y=x3log⁡xx^2 \frac{dy}{dx} + 2x \, y = x^3 \log x

    which is

    ddx(yx2)=x3log⁡x\frac{d}{dx} (y x^2) = x^3 \log x
  5. Step 5: Integrate both sides:

    yx2=∫x3log⁡x dx+Cy x^2 = \int x^3 \log x \, dx + C
  6. Step 6: Evaluate ∫x3log⁡x dx\int x^3 \log x \, dx using integration by parts:

    result=x44log⁡x−x416\text{result} = \frac{x^4}{4} \log x - \frac{x^4}{16}
  7. Step 7: Substitute and solve for yy:

    y=x24log⁡x−x216+Cx2y = \frac{x^2}{4} \log x - \frac{x^2}{16} + \frac{C}{x^2}