Which of the following are set identities?
1) a∪(b∩c) = (a∪b)∩c
2) a ∩ a =∅
3) a∩(a∪b)=a
4) a∩(a∪b)=b
5) a ∪ a = u
6) a ∪ a = a
7) a∩(b∩c) = (a∩b)∩c