y'=[cos(x+y)]'=-sin(x+y)*(1+y')=-sin(x+y)+-sin(x+y)*y'把含y'的部分移到等式的右边，所以：y'=-sin(x+y)/1+sin(x+y)
dy/dx=-sin(x+y)*(1+dy/dx)dy/dy=[-sin(x+y)]/[(1+sin(x+y))]
