待定系数法.令dy/dx=3y/x,dy/y=3dx/x,ln|y|=3ln|x|+C,y=c(x)x^3.dy/dx=dc(x)/dxx^3+3c(x)x^2.将以上两个等式代入原方程得dc(x)/dxx^3+3c(x)x^2-3c(x)x^2=x^3.即dc(x)/dx=1,c(x)=x+c.故y=(x+c)x^3.