9-3a=-3(a-3)a²-9=(a+3)(a-3)分母(9-3a)与(a²-9)的最简公分母是3(a+3)(a-3)2/(9-3a)=2/[-3(a-3)]=-2/[3(a-3)]=-[2(a+3)]/[3(a+3)(a-3)](a-1)/(a²-9)=(a-1)/[(a+3)(a-3)]=[3(a-1)]/[3(a+3)(a-3)]...