（3-a）/(2a-4)÷[(a+2)-5/（a-2)],
=（3-a）/(2a-4)÷[(a²-4-5)/（a-2)],
=（3-a）/(2a-4)÷[(a²-9)/（a-2)],
=（3-a）/(2a-4)×[（a-2)/(a²-9) ],
=（3-a）/[2(a-2)×[（a-2)/(a-3)(a+3) ],
=-1/[2(a+3)] ,
去a=-1,代入原式=-1/4