ylnx=xlny
对x求导
y'*lnx+y*1/x=x'*lny+x*(1/y)*y'
y'*lnx+y/x=lny+y'*x/y
y'(lnx-x/y)=lny-y/x(xlny-y)/x
y'(ylnx-x)/y=(xlny-y)/x
所以y'=y(xlny-y)/[x(ylnx-x)]