∵|a+1/2|+|b-1/2|=0
∴a=-1/2 b=1/2
∴（3/a-7*b）/(a*b)
=[3／(-1/2)-7*1/2]／(-1/2*1/2)
=(-6-7/2)/(-1/4)
=(-19/2)／(-1/4)
=38