$$\xi=\{1,2,\ldots,12\}.$$ $A$ is the set of multiples of $3$ in $\xi$. List $A$ and $A'$.

Question

$$\xi=\{1,2,\ldots,12\}.$$ $A$ is the set of multiples of $3$ in $\xi$. List $A$ and $A'$. Find $n(A')$. Explain why $15\notin A'$.

Accepted Answer

(a) $A=\{3,6,9,12\},\ A'=\{1,2,4,5,7,8,10,11\}$ (b) $8$ (c) $15\notin\xi$

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