Question 91 of 96intermediate🔧 ApplyLong Answer4 marks

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

xdydx+yx+xycotx=0(x0)x \frac{dy}{dx} + y - x + xy \cot x = 0 \quad (x \neq 0)

Correct Answer

The general solution is:

yxsinx=xcosx+sinx+Cy \, x \sin x = -x \cos x + \sin x + C

or equivalently,

y=cotx+1x+Cxsinxy = -\cot x + \frac{1}{x} + \frac{C}{x \sin x}

Exercise: EXERCISE 9.5 | Q: 9 | (Chapter: 30)
For More Understanding

Explanation

This is a linear differential equation of the form dydx+Py=Q\frac{dy}{dx} + Py = Q. After rearranging the given equation and dividing by xx, we identify P=(1x+cotx)P = \left(\frac{1}{x} + \cot x\right) and Q=1Q = 1.

The integrating factor is calculated using ePdxe^{\int P \, dx}, which gives xsinxx \sin x. Using the standard formula for solving linear differential equations, we integrate Q×I.F.Q \times \text{I.F.} to get the final solution.

Solution Steps

  1. Step 1: Rewrite the given equation:

    xdydx+yx+xycotx=0x \frac{dy}{dx} + y - x + xy \cot x = 0

    Dividing by xx (since x0x \neq 0):

    dydx+yx+ycotx=1\frac{dy}{dx} + \frac{y}{x} + y \cot x = 1

  2. Step 2: Rearrange to standard linear form:

    dydx+y(1x+cotx)=1\frac{dy}{dx} + y\left(\frac{1}{x} + \cot x\right) = 1

    Here P=1x+cotxP = \frac{1}{x} + \cot x and Q=1Q = 1

  3. Step 3: Calculate the Integrating Factor (I.F.):

    I.F.=ePdx=e(1x+cotx)dx=elogx+logsinx=elogxsinx=xsinxI.F. = e^{\int P \, dx} = e^{\int \left(\frac{1}{x} + \cot x\right) dx} = e^{\log|x| + \log|\sin x|} = e^{\log|x \sin x|} = x \sin x

  4. Step 4: Apply the general solution formula:

    y×I.F.=Q×I.F.dx+Cy \times I.F. = \int Q \times I.F. \, dx + C

  5. Step 5: Calculate:

    yxsinx=xsinxdx+Cy \cdot x \sin x = \int x \sin x \, dx + C

    Using integration by parts:

    xsinxdx=xcosx+sinx\int x \sin x \, dx = -x \cos x + \sin x

  6. Step 6: Therefore, the general solution is:

    yxsinx=xcosx+sinx+Cy \cdot x \sin x = -x \cos x + \sin x + C

    or

    y=cotx+1x+Cxsinxy = -\cot x + \frac{1}{x} + \frac{C}{x \sin x}