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

  • question_answer
    The solution of the equation \[\frac{dy}{dx}+y\tan x={{x}^{m}}\cos x\] is

    A)                 \[(m+1)y={{x}^{m+1}}\cos x+c(m+1)\cos x\]

    B)                 \[my=({{x}^{m}}+c)\cos x\]

    C)                 \[y=({{x}^{m+1}}+c)\cos x\]

    D)                 None of these

    Correct Answer: A

    Solution :

                       This is the linear equation of the form \[\frac{dy}{dx}+Py=Q\], where \[P=\tan x\] and \[Q={{x}^{m}}\cos x\]                    Now integrating factor (I.F.)\[={{e}^{\int{Pdx}}}={{e}^{\int{\tan dx}}}\]                                                                   \[={{e}^{\log \sec x}}=\sec x\]                    Thus solution is given by, \[y.{{e}^{\int{Pdx}}}=\int{Q}.\,{{e}^{\int{Pdx}}}dx+c\]                    Þ \[y.\sec x=\int{{{x}^{m}}}.\cos x.\sec xdx+c\]Þ\[y\sec x=\frac{{{x}^{m+1}}}{m+1}+c\]                                 Þ \[(m+1)y={{x}^{m+1}}\cos x+c(m+1)\cos x\].


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