JEE Main & Advanced Mathematics Differential Equations Question Bank Variable Separable type differential equations

  • question_answer
    The general solution of the differential equation \[\frac{dy}{dx}+\sin \left( \frac{x+y}{2} \right)=\sin \left( \frac{x-y}{2} \right)\] is   [MP PET 2001]

    A)                 \[\log \tan \left( \frac{y}{2} \right)=c-2\sin x\]   

    B)                 \[\log \tan \,\left( \frac{y}{4} \right)=c-2\sin \left( \frac{x}{2} \right)\]

    C)                 \[\log \tan \,\left( \frac{y}{2}+\frac{\pi }{4} \right)=c-2\sin x\]

    D)  \[\log \tan \left( \frac{y}{4}+\frac{\pi }{4} \right)=c-2\sin \left( \frac{x}{2} \right)\]

    Correct Answer: B

    Solution :

                       \[\frac{dy}{dx}+\sin \left( \frac{x+y}{2} \right)=\sin \left( \frac{x-y}{2} \right)\]         Þ \[\frac{dy}{dx}=\sin \left( \frac{x-y}{2} \right)-\sin \left( \frac{x+y}{2} \right)\]         Þ \[\frac{dy}{dx}=-2\sin \,\left( \frac{y}{2} \right)\,.\cos \,\left( \frac{x}{2} \right)\]         Þ \[\text{cos}\text{ec}\left( \frac{y}{2} \right).dy=-2\cos \left( \frac{x}{2} \right)\,dx\]         Integrating both sides,         \[\int{\text{cosec}\left( \frac{y}{\text{2}} \right)dy=-\int{2\cos \left( \frac{x}{2} \right)dx+c}}\].         Þ \[\frac{\log \,\tan \frac{y}{4}}{1/2}=-\frac{2\sin \left( x/2 \right)}{1/2}+c\]                 Þ \[\log (\tan \frac{y}{4})=c-2\sin (x/2)\].


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