my"+ay^3+by^2+cy=0 y‘=p y''=pdp/dympdp/dy+ay^3+by^2+cy=0mpdp=(ay^3+by^2+cy)dymp^2=ay^4/2+2by^3/3+cy^2+C1y’=p=± √((ay^4/2+2by^3/3+cy^2+C1)/m)∫dy/√((ay^4/2+2by^3/3+cy^2+C1)/m)=±x+C2至于左边的积分...能否用幂级数展开求解当然可以，不过解三次方程也烦