MCQ
If $\cos (x + y) = y\sin x,$ then ${{dy} \over {dx}} = $
  • $ - {{\sin (x + y) + y\cos x} \over {\sin x + \sin (x + y)}}$
  • B
    ${{\sin (x + y) + y\cos x} \over {\sin x + \sin (x + y)}}$
  • C
    $\frac{{y\cos x - \sin (x + y)}}{{\sin x - \sin (x + y)}}$
  • D
    None of these

Answer

Correct option: A.
$ - {{\sin (x + y) + y\cos x} \over {\sin x + \sin (x + y)}}$
a
(a) $\cos (x + y) = (y\sin x)$

==> $ - \sin (x + y)\left( {1 + \frac{{dy}}{{dx}}} \right) = y\cos x + \sin x\frac{{dy}}{{dx}}$

==> $\frac{{dy}}{{dx}} = - \frac{{y\cos x + \sin (x + y)}}{{\sin (x + y) + \sin x}}$.

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