MCQ
If $x = y\sqrt {1 - {y^2},} $ then ${{dy} \over {dx}} = $
  • A
    $0$
  • B
    $x$
  • ${{\sqrt {1 - {y^2}} } \over {1 - 2{y^2}}}$
  • D
    ${{\sqrt {1 - {y^2}} } \over {1 + 2{y^2}}}$

Answer

Correct option: C.
${{\sqrt {1 - {y^2}} } \over {1 - 2{y^2}}}$
c
(c) $x = y\sqrt {1 - {y^2}} $

Differentiate with respect to  $x,$

$1 = \frac{{dy}}{{dx}}\sqrt {1 - {y^2}} + y.\frac{1}{{2\sqrt {1 - {y^2}} }}\,.\,( - 2y)\,.\,\frac{{dy}}{{dx}}$

==> $1 = \frac{{dy}}{{dx}}\sqrt {1 - {y^2}} - \frac{{{y^2}}}{{\sqrt {1 - {y^2}} }}\,.\,\frac{{dy}}{{dx}}$

==> $1 = \frac{{dy}}{{dx}}\left[ {\frac{{1 - {y^2} - {y^2}}}{{\sqrt {1 - {y^2}} }}} \right]$

==> $1 = \frac{{dy}}{{dx}}\left[ {\frac{{1 - 2{y^2}}}{{\sqrt {1 - {y^2}} }}} \right]$

$\frac{{dy}}{{dx}} = \frac{{\sqrt {1 - {y^2}} }}{{1 - 2{y^2}}}$.

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