Inordertolivethekindoflifewewantandtobethepersonwewanttobe，wehavetodomorethanjust________withevents．

The manager is calling on a______ customer trying to talk him into signing the contract ．

The poor countries are extremely _______to international economic fluctuations

Successful learning is not a(n)________activity but consists of four distinct stages in a specific order

设数列 {an} 的前 n 项和为 Sn，则 {an} 等差。

(1) S n4=n 2+2n, n=1,2,3

(2) S n4=n 2+ 2n +1,n=1,2,3