已知a,b互为相反数,则a+b=0
c,b互为倒数,则cd=1
若a不等于0 b=-a b/a=-1
所以3a+3b+b/a+1/2cd
=3(a+b)+(b/a)+(1/2)cd
=3*0+(-1)+(1/2)*1
=-1+1/2
=-1/2
（1/50-1)(1/49-1)(1/48-1)...(1/4-1)(1/3-1)(1/2-1)
=(-49/50)(-48/49)(-47/48).(-3/4)(-2/3)(-1/2)
=-1/50