sin(xy)=x+y 两边对x求导 cos(xy)(y+x*dy/dx)=1+dy/dx
移项变换 dy/dx (cos(xy)-1)=1-ycos(xy)
dy/dx=[1-ycos(xy)]/[xcos(xy)-1]
有 dy={[1-ycos(xy)] / [xcos(xy)-1]}*dx