……这个也需要证明?
|f(x)| ≤ M → -M ≤ f(x) ≤ M,所以有界则既有上界又有下界.
A ≤ f(x) ≤ B → |f(x)| ≤ max{|A|,|B|},所以既有上界又有下界则有界.