第二节书面表达Dear HenryHow is everything going on? I'm writing to invite you to the PhotoExhibition to be held by our Photography Club.The exhibition will last about two weeks from December 22nd to january5th in our lecture hall. All the photos whose topies are concemed with the NewYears Day are welcome. As a student good at photography, you can sharesome of your favorite photos you've ever taken. It's a good chance for you tohow your view about China and help others gain a better understanding ofyou.- If you're interested, you can send your photos lo the Student UnionLooking forward to your early replyLi Hua

17.(1)证明:因为a1,2,2Sn成等差数列,所以3an=25+a1,…………………………………………………当n≥2时,3am-1=2S-1+a1则3a-3am-1=2an,即an=3a-1,即“=3.因为a1=1,所以数列{an}是以1为首项,3为公比的等比数列(2)解:由(1)可得an=a1q=31则aan+1=3-1·3°=3-1(或anan+13则Tn=3+3+35+…+32-1故T=3×[1-(32)”]32+1-31-328(或T=9”+1-924

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