题目

设正项数列的前项和，且满足. （1）求数列的通项公式； （2）若数列，数列的前项和为，求证：. 答案：解：（1）由题意可得，，两式相减得， ，所以， 即，又因为数列为正项数列，所以， 即数列为等差数列，又时，，所以，. （2）由（1）知，又因为， 所以 所以. 完形填空      What do you do at the weekend? Some people like to    1    at home, but others like to go    2    a walk or play football.Mr Jack works hard in factory during the    3   . At the weekend, he always    4    the same thing. On Saturday he    5    hiscar and on    6    he goes with his family to a village by car. His uncle and aunt have a farm there. It isn't a    7    one, butthere is always    8    work to do on the farm. The children help with the animals and give them their    9   . Jack and hiswife help in the fields. At the end of the day, they are all    10    and Jack's aunt gives a big meals. (     )1. A. work(     )2. A. out(     )3. A. day(     )4. A. does(     )5. A. sweeps(     )6. A. Monday(     )7. A. small(     )8. A. much(     )9. A. clothes(     )10.A. early B. sitB. forB. eveningB. hasB. drivingB. SundayB. bigB. manyB. drinksB. late C. stay C. to C. nightC. goes C. sells C. SaturdayC. far C. littleC. food C. hungry D. play     D. away     D. weekdays D. plays    D. washes   D. WednesdayD. long     D. few      D. water    D. full