题目先化简,再求值:(x−1x−x−2x+1)÷2x2−xx2+2x+1(\frac{x-1}{x}-\frac{x-2}{x+1})÷\frac{2x^2-x}{x^2+2x+1}(xx−1−x+1x−2)÷x2+2x+12x2−x,其中x=−12x=-\frac{1}{2}x=−21.知识点：分式的化简求值章节：数与式 / 分式
