原式=1/(a-1)+(1-3a)/[2(a-1)] 通分,最简公分母是2(a-1)
=2/[2(a-1)]+(1-3a)/[2(a-1)]
=(2+1-3a)/[2(a-1)]
=(3-3a)/[2(a-1)]
=-3(a-1)/[2(a-1)],约去(a-1)
=-3/2