Show that the set A = { - 1, 0, 1) is not closed for addition.
For a set to be closed for addition,
For any 2 elements of the set,say a and b, a + b must also be a member of the given set, where a and b may be same or distinct
In the given problem let a = 1 and b = 1
a + b = 2 which is not in the given in set
So the set is not closed for addition.
Hence proved.