RAJASTHAN ­ PET Rajasthan PET Solved Paper-2008

  • question_answer
    The solution of the equation \[x\cos x\left( \frac{dy}{dx} \right)+y(x\sin x+\cos x)=1\]is

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

    B)  \[y=y\text{ }tan\text{ }x+c\]

    C)  \[yx\text{ }sec\text{ }x=tan\text{ }x+c\]

    D)  \[xy\text{ }cos\text{ }x=x+c\]

    Correct Answer: D

    Solution :

     Given, differential equation can be rewitten as \[\frac{dy}{dx}+\left( \frac{x\sin x+\cos x}{x\cos } \right)y=\frac{1}{x\cos x}\] Here,    \[P=\frac{x\sin x+\cos x}{x\cos x}\] and    \[Q=\frac{1}{x\cos x}\] \[\therefore \] \[IF={{e}^{\int{P\,dx}}}\] \[={{e}^{\int{\frac{x\sin x+\cos x}{x\cos x}dx}}}\] \[={{e}^{\log (x\cos x)}}=x\cos x\] \[\therefore \]Required solution is \[y.x\cos x=\int{\frac{x\cos x}{x\cos x}}dx+c\] \[\Rightarrow \] \[xy\text{ }cos\text{ }x=x+c\]


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