听力原文:M: Will you keep an eye on my seat? I'll be back in a minute. W: OK. But I am leaving at 10. You'd better be quick. Q: What do we learn from the conversation? (4)
【简答题】设集合W是满足下列两个条件的无穷数列{a n }的集合:① a n + a n+2 2 ≤a n+1 ,②a n ≤M.其中n∈N + ,M是与n无关的常数. (1)设数列{b n }的通项为b n =5n-2 n ,证明:{b n }∈W; (2)若{a n }是等差数列,S n 是其前n项的和,a 4 =2,S 4 =20,证明:{S n }∈W并求M的取值范围.