MCQ
For differential equation ${y^2}\;dx + \left( {x - \frac{1}{y}} \right)dy = 0\;,y\left( 1 \right) = 1$ then $x = $ 
  • A
    $4 - \frac{2}{y} - \frac{{{e^{\frac{1}{y}}}}}{e}$
  • B
    $\;3 - \frac{1}{y} - \frac{{{e^{\frac{1}{y}}}}}{e}$
  • $\;1 + \frac{1}{y} - \frac{{{e^{\frac{1}{y}}}}}{e}$
  • D
    $\;1 - \frac{1}{y} + \frac{{{e^{\frac{1}{y}}}}}{e}$

Answer

Correct option: C.
$\;1 + \frac{1}{y} - \frac{{{e^{\frac{1}{y}}}}}{e}$
c
Here, $\frac{d x}{d y}+\frac{1}{y^{2}} \cdot x=\frac{1}{y^{3}}$

[linear differential equation in x]

$\therefore \quad {\rm{IF}} = {e^{\int {\frac{1}{{{y^2}}}dy} }} = {e^{ - \frac{1}{y}}}$

Complete solution is

$x \cdot e^{-\frac{1}{y}}=\int \frac{1}{y^{3}} \cdot e^{-\frac{1}{y}} d y$

$\Rightarrow \quad x \cdot e^{-\frac{1}{y}}=\int \frac{1}{y} \cdot \frac{1}{y^{2}} \cdot e^{-\frac{1}{y}} d y$

Put $\quad-\frac{1}{y}=t \Rightarrow \frac{1}{y^{2}} d y=d t$

$\Rightarrow \quad x e^{-\frac{1}{y}}=\int-t \cdot e^{t} d t$

$\Rightarrow \quad x e^{-\frac{1}{y}}=-\left\{t \cdot e^{t}-\int 1 \cdot e^{t} d t\right\}+C$

$\Rightarrow \quad x e^{-\frac{1}{y}}=-t e^{t}+e^{t}+C$

$ \Rightarrow \quad x{e^{ - \frac{1}{y}}} = \frac{1}{y} \cdot {e^{ - \frac{1}{y}}} + {e^{ - \frac{1}{y}}} + C$

$\therefore \quad y(1)=1$

$\therefore \quad e^{-1}=e^{-1}+e^{-1}+C$

$\Rightarrow \quad C=-\frac{1}{e}$

$\therefore \quad x e^{-\frac{1}{y}}=\frac{1}{y} \cdot e^{-\frac{1}{y}}+e^{-\frac{1}{y}}-\frac{1}{e}$

$\Rightarrow \quad x=\frac{1}{y}+1-\frac{1}{e} \cdot e^{\frac{1}{y}}$

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