1-[a- 1/（1-a）]²÷[（a²-a+1）/（a²-2a+1）]
=1-[a(1-a)/(1-a)-1/(1-a)]²÷[（a²-a+1）/（a-1）²]
=1-(a²-a+1)²/(a-1)²×（a-1）²/（a²-a+1）
=1-(a²-a+1)
=-a²+a
愿对你有所帮助!