Q117 / 180 · COMEDK 2025 (10 May Shift 1) · Mathematics
The solution of the differential equation $\dfrac{d y}{d x}+y \log y \cot x=0$ is
- A$\cos x \log y=c$
- Correct answerB$\sin x \log y=c$
- C$\log y=c \sin x$
- D$y \sin x=c$
Solution
The given differential equation is $\dfrac{dy}{dx} + y \log y \cot x = 0$.
Rearranging the terms, we get $\dfrac{dy}{y \log y} = -\cot x dx$.
Integrating both sides, we have $\int \dfrac{dy}{y \log y} = -\int \cot x dx$.
Let $u = \log y$, then $du = \dfrac{1}{y} dy$. The integral becomes $\int \dfrac{du}{u} = -\int \cot x dx$.
This results in $\log |u| = -\log |\sin x| + C_1$.
Substituting $u = \log y$, we get $\log |\log y| = -\log |\sin x| + C_1$.
This simplifies to $\log |\log y| + \log |\sin x| = C_1$, which is $\log |\log y \sin x| = C_1$.
Taking the exponential of both sides, $|\log y \sin x| = e^{C_1} = C$.
Thus, $\sin x \log y = C$.
Answer: $\sin x \log y=c$
Rearranging the terms, we get $\dfrac{dy}{y \log y} = -\cot x dx$.
Integrating both sides, we have $\int \dfrac{dy}{y \log y} = -\int \cot x dx$.
Let $u = \log y$, then $du = \dfrac{1}{y} dy$. The integral becomes $\int \dfrac{du}{u} = -\int \cot x dx$.
This results in $\log |u| = -\log |\sin x| + C_1$.
Substituting $u = \log y$, we get $\log |\log y| = -\log |\sin x| + C_1$.
This simplifies to $\log |\log y| + \log |\sin x| = C_1$, which is $\log |\log y \sin x| = C_1$.
Taking the exponential of both sides, $|\log y \sin x| = e^{C_1} = C$.
Thus, $\sin x \log y = C$.
Answer: $\sin x \log y=c$
Hence, the correct answer is option: 'B'.