The solution of (1) set must satisfy the certainty, mutual difference and disorder. In six examples, (6) does not satisfy the certainty, and it is not clear how high it is, so 12345 can form a set.

(2) There are two methods to describe a set: enumeration method and description method. You can list the elements of the collection one by one, or you can use the description method "{representing the nature of elements | satisfaction}".

(3) The elements in a finite set are finite. Where 125 is a finite set.

Answer: (1) 12345(2) See (3) 125 above.