d(x^2+y^2+x)/dy=2yd(xy)/dx=y(2y-y)/xy=1/xe^[∫(1/x)dx]=x（积分因子）(x^3+xy^2+x^2)dx+yx^2dy=0x^4/4+x^3/3+(y^2)d(x^2)/2+(x^2)d(y^2)/2=0x^4/4+x^3/3+(x^2)(y^2)/2=C
