=[a-(2a-1)/a]÷(1-a²)/(a²+a)
=(a²-2a+1)/a×a(a+1)/[-(a+1)(a-1)]
=(a-1)²/a×[-a/(a-1)]
=-(a-1)
=1-a
当a=2时 原式=1-a=1-2=-1
注意a不要取0 1 -1 三个值