（x-1）分之（x²-2x）÷【（x+1）分之（2x-1）-x+1】=（x-1）分之（x²-2x）÷【（x+1）分之（2x-1-x²+1）】=（x-1）分之（x²-2x）÷【（x+1）分之（2x-x²）】=（x-1）分之（x²-2x）* [-(x²-2x）分之(x+1)]=-[(x-1)分之(x+1)]
