MCQ
The solution curve of the differential equation $y \frac{d x}{d y}=x\left(\log _e x-\log _e y+1\right), x>0, y>0 \text { passing }$ through the point$(\mathrm{e}, 1)$ is
  • A
    $\left|\log _e \frac{y}{x}\right|=x$
  • B
     $\left|\log _e \frac{y}{x}\right|=y^2$
  • $\left|\log _e \frac{x}{y}\right|=y$
  • D
    $2\left|\log _e \frac{x}{y}\right|=y+1$

Answer

Correct option: C.
$\left|\log _e \frac{x}{y}\right|=y$
c
$\frac{d x}{d y}=\frac{x}{y}\left(\ln \left(\frac{x}{y}\right)+1\right)$

$\text { Let } \frac{x}{y}=t \Rightarrow x=t y$

$\frac{d x}{d y}=t+y \frac{d t}{d y}$

$t+y \frac{d t}{d y}=t(\ln (t)+1)$

$y \frac{d t}{d y}=t \ln (t) \Rightarrow \frac{d t}{t \ln (t)}=\frac{d y}{y}$

$\Rightarrow \int \frac{d t}{t \cdot \ln (t)}=\int \frac{d y}{y}$

$\Rightarrow \int \frac{d p}{p}=\int \frac{d y}{y} \quad$ $\text { let } \ln t=p$

$\Rightarrow \ln p=\ln y+c$

$\ln (\ln t)=\ln y+c$

$\ln \left(\ln \left(\frac{x}{y}\right)\right)=\ln y+c$

$\text { at } x=e, y=1$

$\ln \left(\ln \left(\frac{e}{1}\right)\right)=\ln (1)+c \Rightarrow c=0$

$\ln \left|\ln \left(\frac{x}{y}\right)\right|=\ln y$

$\left|\ln \left(\frac{x}{y}\right)\right|=e^{\ln y}$

$\left|\ln \left(\frac{x}{y}\right)\right|=y$

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