Mathematics, 22.07.2021 08:50 bbygrill
Chứng minh rằng nếu $A \setminus B=A$ thì $A\cap B=\emptyset$.
Giải: Giả sử $A\cap B\neq \emptyset \Rightarrow \exists x \in A \cap B$. Suy ra $x \in B \, ^{(1)}$ và $x \in A \, ^{(2)}$. Từ $(1)$ suy ra $x \notin A \setminus B$. Mà $A \setminus B=A$ nên $x \notin A$ (mâu thuẫn với (2)).
Phương pháp chứng minh nào đã được sử dụng?
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b. Trực tiếp
c. Tầm thường
d. Phản chứng
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Chứng minh rằng nếu $A \setminus B=A$ thì $A\cap B=\emptyset$.
Giải: Giả sử $A\cap B\neq \emptyset...
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