MCQ
If ${e^y} + xy = e$, the ordered pair $\left( {\frac{{dy}}{{dx}},\frac{{{d^2}y}}{{d{x^2}}}} \right)$ at $x = 0$ is equal to
  • A
    $\left( {\frac{1}{e}, - \frac{1}{{{e^2}}}} \right)$
  • B
    $\left( {\frac{1}{e},  \frac{1}{{{e^2}}}} \right)$
  • $\left( { - \frac{1}{e},\frac{1}{{{e^2}}}} \right)$
  • D
    $\left( { - \frac{1}{e}, - \frac{1}{{{e^2}}}} \right)$

Answer

Correct option: C.
$\left( { - \frac{1}{e},\frac{1}{{{e^2}}}} \right)$
c
${e^y} = xy = e$ differentiate w.r.t. $x$

${e^y}\frac{{dy}}{{dx}} + x\frac{{dy}}{{dx}} + y = 0$

${e^y}\frac{{dy}}{{dx}} + x\frac{{dy}}{{dx}} + y = 0$

$\frac{{dy}}{{dx}}\left( {x + {e^y}} \right) =  - y,{\left. {\frac{{dy}}{{dx}}} \right|_{\left( {0,1} \right)}} =  - \frac{1}{e}$ again differentate w.r.t. $x$

${e^y}.\frac{{{d^2}y}}{{d{x^2}}} + \frac{{dy}}{{dx}}.{e^y}.\frac{{dy}}{{dx}} + x.\frac{{{d^2}y}}{{d{x^2}}} + \frac{{dy}}{{dx}} + \frac{{dy}}{{dx}} = 0$

$\left( {x + {e^y}} \right)\frac{{{d^2}y}}{{d{x^2}}} + {\left( {\frac{{dy}}{{dx}}} \right)^2}.{e^y} + 2\frac{{dy}}{{dx}} = 0$

$e\frac{{{d^2}y}}{{d{x^2}}} + \frac{1}{{{e^1}}}e + 2\left( { - \frac{1}{e}} \right) = 0$

$\therefore \frac{{{d^2}y}}{{d{x^2}}} + \frac{1}{{{e^2}}}$

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