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\[ x^{2} \frac{d^{2} y}{d x^{2}}+5 x \frac{d y}{d x}-5 y=2 \log x \text {. } \] By cauchy's Homogeneous method. |
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Answer» Given differential equation is x2 \(\frac{d^2y}{dx^2}+5x\frac{dy}{dx}-5y=2log x\) Let x = ez then x\(\frac{dy}{dx}=\frac{dy}{dz}\) = Dy, where \(\frac{d}{dx}=D\) and \(x^2\frac{d^2y}{dx^2}\) = D(D - 1)y \(\therefore\) Given differential equation converts into D(D - 1)y + 5Dy - 5y = 2z (\(\because\) log x = z) ⇒ (D2 - D + 5D - 5)y = 2z ⇒ (D2 + 4D - 5)y = 2z----(1) It's auxilarly equation is m2 + 4m - 5 = 0 ⇒ (m + 5)(m - 1) = 0 ⇒ m + 5 = 0 or m - 1 = 0 ⇒ m = -5 or m = 1 \(\therefore\) C.F. = C1e-5z + C2ez & P.I. = \(\frac{1}{D^2+4D-5}2z\) \(=\frac{-2}5\cfrac1{1-\frac{D^2+4D}5}z\)
\(=\frac{-2}5(1-\frac{D^2+4D}5)^{-1}z\) \(=\frac{-2}5(1+\frac{D^2+4D}5+\frac{(D^2+4D)^2}{5^2}+....)z\) (\(\because\) (1 - x)-1 = 1 + x + x2+....) \(=\frac{-2}5(z+\frac15D^2z+\frac45 Dz)\) \(=\frac{-2}5(z+0+\frac45)\) (\(\because\) Dz = \(\frac{dz}{dz}=1\)) \(=\frac{-2}5z-\frac8{25}\) \(\therefore\) Complete solution of differential equation(1) is y = C.F. + P. I. = C1(ez)-5 + C2ez- \(\frac25z-\frac8{25}\) = C1x-5 + C2 x - \(\frac25\) log x - \(\frac8{25}\) (\(\because\) ez = x ⇒ z = log x) = \(\frac{C_1}{x^5}+C_2x - \frac25logx-\frac8{25}\) which is solution of given differential equation. |
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