Def 8.5 [𝑎,𝑏] is sequentially closed imply (∃𝑥=lim𝑛(𝑥𝑛))(𝑎≤𝑥𝑛≤𝑏)⟹𝑎≤𝑥≤𝑏⟹𝑥∈[𝑎,𝑏] Basically, all limits of it’s sub-sequences are in itself.