JEE Main & Advanced Mathematics Differential Equations Question Bank Formation of differential equations

  • question_answer
    If \[x=\sin t\], \[y=\cos pt\], then          [Karnataka CET 2005]

    A)                 \[(1-{{x}^{2}}){{y}_{2}}+x{{y}_{1}}+{{p}^{2}}y=0\]

    B)                 \[(1-{{x}^{2}}){{y}_{2}}+x{{y}_{1}}-{{p}^{2}}y=0\]

    C)                 \[(1+{{x}^{2}}){{y}_{2}}-x{{y}_{1}}+{{p}^{2}}y=0\]

    D)             \[(1-{{x}^{2}}){{y}_{2}}-x{{y}_{1}}+{{p}^{2}}y=0\]

    Correct Answer: D

    Solution :

                       \[x=\sin t\], \[y=\cos pt\]                    \[\frac{dx}{dt}=\cos t\]; \[\frac{dy}{dt}=-p\sin pt\]; \[\frac{dy}{dx}=\frac{-p\sin pt}{\cos t}\]                    \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=\frac{-\cos t\,{{p}^{2}}\cos pt(dt/dx)-p\sin pt\sin t(dt/dx)}{{{\cos }^{2}}t}\]                    Þ \[(1-{{x}^{2}})\frac{{{d}^{2}}y}{d{{x}^{2}}}-x\frac{dy}{dx}+{{p}^{2}}y=0\]                 or \[(1-{{x}^{2}}){{y}_{2}}-x{{y}_{1}}+{{p}^{2}}y=0\].


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