∵ a,b 互为相反数,c、d 互为倒数
∴ a + b = 0 ,cd = 1
∴ 6 - 2 a - 2 b + 3cd
= 6 - 2（a + b）+ 3 × 1
= 6 - 2 × 0 + 3
= 6 + 3
= 9