You can understand that less than 3 is equivalent to a set, and less than or equal to 3 is also a set, so it can be concluded that a set less than 3 is a proper subset of a set less than or equal to 3, so if [sn x s (n+2)]/s (n+ 1) 2 is less than or equal to 3, it must be less than or equal to 3.