求yy"=(y')^2-(y')^3的通解
p(ydp/dy-p+p²)=0∴p=0.(1)ydp/dy-p+p²="}}}'>

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  • 设y'=p,则y''=pdp/dy

    代入原方程得ypdp/dy=p²-p³

    ==>p(ydp/dy-p+p²)=0

    ∴p=0.(1)

    ydp/dy-p+p²=0.(2)

    由(1)得,原方程的一个解是y=C (C是积分常数)

    由(2)得,ydp/dy=p-p²

    ==>dp/(p-p²)=dy/y

    ==>ln│p/(1-p)│=ln│y│+ln│C1│ (C1是非零积分常数)

    ==>p/(1-p)=C1y

    ==>p=C1y/(C1y+1)

    ==>y'=C1y/(C1y+1)

    ==>(1+1/(C1y))dy=dx

    ==>y+ln│y│/C1=x+ln│C2│/C1 (C2是非零积分常数)

    ==>y=C2e^(C1(x-y))

    即原方程另一解是y=C2e^(C1(x-y)) (C1,C2是非零积分常数)

    故 综合上述所有解知,原方程的通解是y=C2e^(C1(x-y)) (C1,C2是积分常数).