2015数学一中考答案

设u=y/x，则y=ux，dy/dx=u+x(du/dx)。

By dy/dx=-y？/(2xy+y？)=-y/(2x+y)

= = & gtu+x(du/dx)=-ux/(2x+ux)

= = & gtu+x(du/dx)=-u/(2+u)

= = & gt-x(du/dx)=u+[u/(2+u)]=(u？+3u)/(2+u)

= = & gt[(2+u)/(u？+3u)]du=(-1/x)dx