d(xy)+d(㏑y)+d(㏑x)=d(1)y*dx+x*dy+1/y*dy+1/x*dx=0(x+1/y)dy=(-y-1/x)dxdy/dx=(-y-1/x)/(x+1/y)x=1代入xy+㏑y+㏑x=1y+㏑y+0=1y=1所以dy/dx∣x=1=(-1-1)/(1+1)=-1
