a,b互为倒数,
ab=1
c,d互为相反数,
c+d=0
|x|=2
x=±2
当x=2时
3ab+(c+d)/x+2x
=3*1+0/2+2*2
=3+0+4
=7
当x=-2时
3ab+(c+d)/x+2x
=3*1+0/(-2)+2*(-2)
=3+0-4
=-1