Find the smallest set A such that

We have to find the smallest set A such that A∪1, 2=1, 2, 3, 5, 9">A∪{1, 2}={1, 2, 3, 5, 9}A∪1, 2=1, 2, 3, 5, 9.

The union of the two sets A & B is the set of all those elements that belong to A or to B or to both A & B.

Thus, A must be {3, 5, 9}.