(7/2a-4)-(3/2+ a)-(12/a平方-4)
=7/2(a-2)-3/(a+2)-12/(a+2)(a-2)
=[7(a+2)-6(a-2)-24]/2(a+2)(a-2)
=(a+2)/2(a+2)(a-2)
=1/2(a-2)
=1/2(-1-2)
=-1/6