Q99 / 180 · COMEDK 2022 · Mathematics
The solution of the differential equation $x \dfrac{d y}{d x}=\cot y$ is
- A$y \cos x=c$
- Correct answerB$x \cos y=c$
- C$\log (x \cos y)=c$
- D$\log (y \cos x)=c$
Solution
The given differential equation is $x \dfrac{dy}{dx} = \cot y$.
Separating the variables, we get $\dfrac{dy}{\cot y} = \dfrac{dx}{x}$.
This simplifies to $\tan y \, dy = \dfrac{dx}{x}$.
Integrating both sides, we have $\int \tan y \, dy = \int \dfrac{dx}{x}$.
The integral of $\tan y$ is $\ln |\sec y|$ or $-\ln |\cos y|$.
Thus, $-\ln |\cos y| = \ln |x| + C$.
Rearranging the terms, $\ln |x| + \ln |\cos y| = -C$.
Using the property of logarithms, $\ln |x \cos y| = C'$, where $C' = -C$.
Taking the exponential of both sides, $x \cos y = e^{C'} = c$.
Answer: $x \cos y=c$
Separating the variables, we get $\dfrac{dy}{\cot y} = \dfrac{dx}{x}$.
This simplifies to $\tan y \, dy = \dfrac{dx}{x}$.
Integrating both sides, we have $\int \tan y \, dy = \int \dfrac{dx}{x}$.
The integral of $\tan y$ is $\ln |\sec y|$ or $-\ln |\cos y|$.
Thus, $-\ln |\cos y| = \ln |x| + C$.
Rearranging the terms, $\ln |x| + \ln |\cos y| = -C$.
Using the property of logarithms, $\ln |x \cos y| = C'$, where $C' = -C$.
Taking the exponential of both sides, $x \cos y = e^{C'} = c$.
Answer: $x \cos y=c$
Hence, the correct answer is option: 'B'.