(1)(x+5)+b=x+(5+b);
(2)8x+6x=(8+6)×x;
(3)5a+3a+2a=(5+3+2)×a;
(4)(a•b)•c=a•(b•c);
故答案为:(1)5,b;
(2)8,6,x;
(3)5,3,2,a;
(4)a,b,c.
(1)(x+5)+b=x+(5+b);
(2)8x+6x=(8+6)×x;
(3)5a+3a+2a=(5+3+2)×a;
(4)(a•b)•c=a•(b•c);
故答案为:(1)5,b;
(2)8,6,x;
(3)5,3,2,a;
(4)a,b,c.