二阶微分方程求一般表达式
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二阶微分方程求一般表达式
ty′′ = y′(ln y′− ln t)
ty′′ = y′(ln y′− ln t)
∵令z=y'/t,则y'=tz,y''=tz'+z
∴ty′′= y′(ln y′−ln t) ==>ty′′= y′ln(y'/t)
==>t(tz'+z)=tzlnz
==>tz'+z=zlnz
==>tz'=z(lnz-1)
==>dz/(z(lnz-1))=dt/t
==>d(lnz-1)/(lnz-1)=dt/t
==>ln│lnz-1│=ln│t│+ln│C1│ (C1是非零积分常数)
==>lnz-1=C1t
==>z=e^(C1t+1)
==>y'/t=e^(C1t+1)
==>y'=te^(C1t+1)
==>y=∫te^(C1t+1)=(t/C1-1/C1²)e^(C1t+1)+C2 (C2是积分常数)
故原方程的通解是y=(t/C1-1/C1²)e^(C1t+1)+C2.
∴ty′′= y′(ln y′−ln t) ==>ty′′= y′ln(y'/t)
==>t(tz'+z)=tzlnz
==>tz'+z=zlnz
==>tz'=z(lnz-1)
==>dz/(z(lnz-1))=dt/t
==>d(lnz-1)/(lnz-1)=dt/t
==>ln│lnz-1│=ln│t│+ln│C1│ (C1是非零积分常数)
==>lnz-1=C1t
==>z=e^(C1t+1)
==>y'/t=e^(C1t+1)
==>y'=te^(C1t+1)
==>y=∫te^(C1t+1)=(t/C1-1/C1²)e^(C1t+1)+C2 (C2是积分常数)
故原方程的通解是y=(t/C1-1/C1²)e^(C1t+1)+C2.