JEE Main & Advanced Mathematics Differential Equations Question Bank Linear defferential equations

  • question_answer
    The solution of \[dy=\cos x(2-y\cos \text{ec}x)dx\] where \[y=2\] when \[x=\frac{\pi }{2}\] is [J & K 2005]

    A)                 \[y=\sin x+\text{cosec }x\] 

    B)                 \[y=\tan \frac{x}{2}+\cot \frac{x}{2}\]

    C)                 \[y=\frac{1}{\sqrt{2}}\sec \frac{x}{2}+\sqrt{2}\cos \frac{x}{2}\]

    D)                 None of these

    Correct Answer: A

    Solution :

                       \[\frac{dy}{dx}=2\cos x-y\cot x\] Þ \[\frac{dy}{dx}+y\cot x=2\cos x\] I.F.\[={{e}^{\int{\cot xdx}}}=\sin x\] \[y.\sin x=\int{2\cos x.\sin x+c}\] \[y\sin x={{\sin }^{2}}x+c\] at \[y=2\] and \[x=\frac{\pi }{2}\], \[c=1\]; \[y=\sin x+\cos \text{ec}x\]        


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