原式=[1/(x-3)(x-4) +2/(x-1)(x-3) ]×(x-4)(x-1)/3x
=[(x-1)+2(x-4)]/(x-1)(x-3)(x-4) ×(x-4)(x-1)/3x
=(3x-9)/(x-3) ×1/3x
=3(x-3)/(x-3) ×1/3x
=1/x