a b互为相反数,则a+b=0,
m,n互为倒数,则mn=1,
(-2a-b+3)/(2mn)+(-1/2b)
=(-2a-b+3)/2+(-1/2b)
=-a-1/2b+3/2-1/2b
=-a-b+3/2
=-(a+b)+3/2
=0+3/2
=3/2