a.b互为相反数,得a+b=0
c.d互为倒数.得cd=1
m的绝对值是5,得m=5 或-5
3a-cd/2+3b+m-3
=3a-1/2+3b+m-3
=3(a+b)+m-3.5
=m-3.5
(1)m=5,原式=5-3.5=1.5
(2)m=-5,原式=-5-3.5=-8.5