题目

Mrs. Green is going to give a birthday party for Mary. Mary is her daughter. She is going to be ten years old. A lot of friends of Mary’s are going to come to the party. There are twenty girls of them. Mrs. Green is getting ready for the party. Mrs. White is helping her. “That’s a big nice cake,” Mrs. White says to Mrs. Green. “Thank you very much.” Mrs. Green is going shopping now. She is going to buy fruit for the party. She buys lots of pears, apples, oranges and bananas. Then she goes home. It’s five o’clock in the afternoon. Everything is ready. Now the first girl 已知数列{an}满足：a1=6，an+1=n+2nan+(n+1)(n+2)，（1）求a2，a3；（2）若dn=ann(n+1)，求数列{dn}的通项公式；（3）若an=kC3n+2，（其中Cnm表示组合数），求数列{an}的前n项和Sn．