Question 74 of 96intermediate🔧 ApplyLong Answer4 marks

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

y dx+(x−y2) dy=0y \, dx + (x - y^2) \, dy = 0
Correct Answer

The general solution is:

xy=y33+Cxy = \frac{y^3}{3} + C

or alternatively:

x=y23+Cyx = \frac{y^2}{3} + \frac{C}{y}
Exercise: EXERCISE 9.5 | Q: 11 | (Page 30)
For More Understanding

Explanation

The provided question asks for the general solution of the differential equation y dx+(x−y2) dy=0y \, dx + (x - y^2) \, dy = 0. Based on the context provided, this question falls under Exercise 9.5, which deals with linear differential equations of the form dydx+Py=Q\frac{dy}{dx} + Py = Q or dxdy+Px=Q\frac{dx}{dy} + Px = Q.

The equation is rearranged into the linear form:

dxdy+1yx=y\frac{dx}{dy} + \frac{1}{y}x = y

to find the integrating factor and subsequently the general solution.

Solution Steps

  1. Step 1: Rearrange the given equation.

    The given differential equation is:

    y dx+(x−y2) dy=0y \, dx + (x - y^2) \, dy = 0

    Rearranging the terms, we get:

    y dx=−(x−y2) dyy \, dx = -(x - y^2) \, dy y dx=(y2−x) dyy \, dx = (y^2 - x) \, dy

    Dividing both sides by dydy (and assuming y≠0y \neq 0), we write it in terms of dxdy\frac{dx}{dy}:

    ydxdy=y2−xy \frac{dx}{dy} = y^2 - x ydxdy+x=y2y \frac{dx}{dy} + x = y^2
  2. Step 2: Convert to standard linear form.

    Divide the equation by yy to get the coefficient of dxdy\frac{dx}{dy} as 1:

    dxdy+1yx=y\frac{dx}{dy} + \frac{1}{y}x = y

    This is a linear differential equation of the form dxdy+Px=Q\frac{dx}{dy} + Px = Q, where:

    P=1yandQ=yP = \frac{1}{y} \quad \text{and} \quad Q = y
  3. Step 3: Find the Integrating Factor (I.F.).

    The Integrating Factor is given by e∫P dye^{\int P \, dy}.

    I.F.=e∫1y dy=eln⁡∣y∣=yI.F. = e^{\int \frac{1}{y} \, dy} = e^{\ln|y|} = y
  4. Step 4: Write the general solution.

    The solution of a linear differential equation is given by:

    x(I.F.)=∫Q(I.F.) dy+Cx(I.F.) = \int Q(I.F.) \, dy + C

    Substituting the values:

    x(y)=∫y(y) dy+Cx(y) = \int y(y) \, dy + C xy=∫y2 dy+Cxy = \int y^2 \, dy + C xy=y33+Cxy = \frac{y^3}{3} + C
  5. Step 5: Final Answer.

    The general solution is:

    xy=y33+Cxy = \frac{y^3}{3} + C

    ∴ xy=y33+Cxy = \frac{y^3}{3} + C