a与b互为相反数,a+b=0
c与d互为倒数,cd=1
x的绝对值等于2.,x=±2
①x=2时
2（a+b）-x+cd/3
=2*0-2+1/3
=-5/3
②x=-2时
2（a+b）-x+cd/3
=2*0-(-2)+1/3
=7/3