（x-1/x+1 - x+1/x-3 ）除 2/x²+2x+1

[(x-1)/(x+1) - (x+1)/(x-3)]÷ 2/(x²+2x+1)=[(x-1)(x-3)-(x+1)²]/[(x+1)(x-3)]乘(x+1)²/2=(x²-4x+3-x²-2x-1)/[(x+1)(x-3)]乘(x+1)²/2=(-6x+2)/[(x+1)(x-3)]乘(x+1)²/2=-2(3x-1)...