y=(cosx+2)/(sinx-1)ysinx-y=cosx+2ysinx-cosx=y+2√(y²+1)sin(x-t)=y+2,t=arctan(1/y)sin(x-t)=(y+2)/√(y²+1)故|y+2|/√(y²+1)≤1y²+4y+4≤y²+1y≤-3/4此即为值域.