因为 y*sinx-cos(x-y)=0
则：d(ysinx)-dcos(x-y)=0
即：sinxdy+dcosxdx+sin(x-y)(dx-dy)=0
整理得：[sin(x-y)-sinx]dy=[ycosx+sin(x-y)]dx
所以：dy=[ycosx+sin(x-y)]dx/[sin(x-y)-sinx]