[a- 2a/(a+1 ）]÷ (a²-2a+1)/(a²-1) -a
=a(a-1)/(a+1)÷ (a-1)²/(a+1)(a-1)-a
=a(a-1)/(a+1)÷ (a-1)/(a+1)-a
=a-a
=0