题目

1.      You’ve lost your hat? Well, that’s not much to __________. A. worry       B. worry about              C. worry for  D. worry at   答案：B5．已知数列{an}为等差数列，a2=3，a4=7；数列{bn}为公比为q（q＞1）的等比数列，且满足集合{b1，b2，b3}={1，2，4}．（Ⅰ）求数列{an}，{bn}的通项公式；（Ⅱ）求数列{an+bn}的前n项和Sn．