Exercise 9. Let (X,d) be a metric space and let A, B be two subsets of X. Prove that \operatorname{Fr}(A \cup B) \subseteq \operatorname{Fr}(A) \cup \operatorname{Er}(B) \text { Show also that if } \bar{A} \cap \bar{B}=\emptyset, \text { then } \operatorname{Fr}(A \cup B)=\operatorname{Fr}(A) \cup \operatorname{tr}(B)

Fig: 1

Fig: 2

Fig: 3