Write down all the subsets of the following set: $\{a, b\}$

Let $A = \{1, 2, 3, 4, 5, 6\}$. Insert the appropriate symbol $\in$ or $\notin$ in the blank space:
$4 \dots A$

Two sets $A$ and $B$ are disjoint if and only if

State whether the following statement is true or false. Justify your answer.
${2, 6, 10}$ and ${3, 7, 11}$ are disjoint sets.

In each of the following,determine whether the statement is true or false. If it is true,prove it. If it is false,give an example.
If $A \subset B$ and $x \notin B,$ then $x \notin A$.

State whether the following set is finite or infinite:
The set of circles passing through the origin $(0,0)$.