MCQ
The solution of the equation $\frac{d y}{d x}=\cos ^2 y$ is :
  • A
    $x+\tan y=c$
  • $\tan y=x+c$
  • C
    $\sin y+x=c$
  • D
    $\sin y-x=c$

Answer

Correct option: B.
$\tan y=x+c$
(B)
$
\begin{aligned}
\frac{d y}{d x} & =\cos ^2 y \\
\Rightarrow \quad \frac{1}{\cos ^2 y} d y & =d x \Rightarrow \sec ^2 y d y=d x
\end{aligned}
$
Hence
$
\begin{aligned}
\int \sec ^2 y d y & =\int d x \\
\tan y & =x+c
\end{aligned}
$
Therefore the correct choice is (B).

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