Find the smallest set A such that A ∪ {1, 2} = {1, 2, 3, 5, 9}.
A ∪ {1, 2} = {1, 2, 3, 5, 9}
Elements of A and {1, 2} together give us the result
So smallest se of A can be
A = {1, 2, 3, 5, 9} – {1, 2}
A = {3, 5, 9}.