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

  • question_answer
    The solution of the differential equation \[(\sin x+\cos x)dy+(\cos x-\sin x)dx=0\]is

    A)                 \[{{e}^{x}}(\sin x+\cos x)+c=0\]   

    B)                 \[{{e}^{y}}(\sin x+\cos x)=c\]

    C)                 \[{{e}^{y}}(\cos x-\sin x)=c\]           

    D)                 \[{{e}^{x}}(\sin x-\cos x)=c\]

    Correct Answer: B

    Solution :

                       \[\frac{dy}{dx}=-\frac{\cos x-\sin x}{\sin x+\cos x}\] Þ \[dy=-\left( \frac{\cos x-\sin x}{\sin x+\cos x} \right)dx\]         On integrating both sides, we get         Þ \[y=-\log (\sin x+\cos x)+\log c\]                                 Þ \[y=\log \left( \frac{c}{\sin x+\cos x} \right)\] Þ \[{{e}^{y}}(\sin x+\cos x)=c\].


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