so i just started learning this subject. i undestand the basics of it but can't seem to understand how to prove a curtain claim on it.
for example i received a question as homework that i would really appreaciate it if someone could help me with it, please explain it step by step:
- $A \subset \mathbb R$ has an upper bound.
- $B = \{ -a \mid a \in A\}$
prove that $B$ has a lower bound and that $\inf B = -\sup A$.
please help.. thank you.