Question 56 of 96intermediate🔧 ApplyLong Answer4 marks

In each of the Exercises 1 to 10, show that the given differential equation is homogeneous and solve each of them.

(1+exy)dx+exy(1−xy)dy=0\left(1 + e^{\frac{x}{y}}\right) dx + e^{\frac{x}{y}} \left(1 - \frac{x}{y}\right) dy = 0
Correct Answer

The general solution is:

x+yexy=Cx + ye^{\frac{x}{y}} = C

where CC is an arbitrary constant.

Exercise: EXERCISE 9.4 | Q: 10 | (Page 22)
For More Understanding

Explanation

This is Exercise 9.4, Question 10 from NCERT Class 12 Mathematics. The question requires showing the differential equation is homogeneous and then solving it using the standard substitution method for homogeneous equations.

Solution Steps

  1. Step 1: Rewrite the given equation as:

    dxdy=−exy(1−xy)1+exy\frac{dx}{dy} = \frac{-e^{\frac{x}{y}}\left(1 - \frac{x}{y}\right)}{1 + e^{\frac{x}{y}}}

    Since the right-hand side is a function of xy\frac{x}{y} alone, the equation is homogeneous.

  2. Step 2: Substitute x=vyx = vy, so dxdy=v+ydvdy\frac{dx}{dy} = v + y\frac{dv}{dy}. This gives:

    v+ydvdy=−ev(1−v)1+evv + y\frac{dv}{dy} = \frac{-e^{v}(1-v)}{1+e^{v}}
  3. Step 3: Simplify to get:

    ydvdy=−(ev+v)1+evy\frac{dv}{dy} = \frac{-(e^{v}+v)}{1+e^{v}}

    Separating variables:

    1+evev+v dv=−dyy\frac{1+e^{v}}{e^{v}+v}\,dv = -\frac{dy}{y}
  4. Step 4: Integrate both sides: Let u=ev+vu = e^{v}+v, so du=(1+ev) dvdu = (1+e^{v})\,dv. Thus:

    ∫duu=−∫dyy\int\frac{du}{u} = -\int\frac{dy}{y}

    giving:

    ln⁡∣ev+v∣=−ln⁡∣y∣+ln⁡C\ln|e^{v}+v| = -\ln|y| + \ln C
  5. Step 5: Simplify to get:

    y(ev+v)=Cy(e^{v}+v) = C

    Substituting back v=xyv = \frac{x}{y}:

    y(exy+xy)=Cy\left(e^{\frac{x}{y}} + \frac{x}{y}\right) = C

    or:

    x+yexy=Cx + ye^{\frac{x}{y}} = C

    ∴ The general solution is x+yexy=Cx + ye^{\frac{x}{y}} = C.