Question

Find the smallest set A such that $A\cup \left\{1,2\right\}=\left\{1,2,3,5,9\right\}$.

Answer 1

We have to find the smallest set A such that $A\cup \left\{1,2\right\}=\left\{1,2,3,5,9\right\}$.

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}.