1/(a²+a)与 1/(a²－1) 通分,公分母是(a³-a)1/(a²+a )--------------------(a²+a)是分母=1/[a(a+1)]=(a-1)/[a(a+1)(a-1)]=(a-1)/(a³-a)1/(a²－1) --------------------(a²－...