17.解:(1)记数列{an}的公差为d,而a3=a1a,-(1分)即(a1+3d)2=(a1+d)(a1+7d),(2分)即a3+6a1d+9d2=a1+8a1d+7d2,故d2-a1d=0,得d=0或d=a1,(3分)因为S20=420,即20a1+190d=420,即2a1+19d=42,4分)当d=0时,a1=21;当d=a1时,a1=2故数列{an}的通项公式为an=21或an=2n,(6分)(2)由(1)可知,an=2n,(7分)1则口a,+1)(a++1)(2n+1)(2n++5-2(2n+1(9分+3故T(3-3)+(5-7)++2n+12n+3-2…十2n+1-2n+3)=2(32n+3)6n+9·(12分)

读后续写 One possible version)A bad guy just p/cked Kim up and took her away! yelled Julie. So shocked were Kims parents thatthey jumped up from the sofa and rushed over to Julie. Learning what had happened, they phoned thepolice without delay. The nextnt, they jumped into their car and drove off directed by Julie. By thenT had reached the alley, but the man was nowhere to be seen. "I must find Kim, "TJ said to himself. Heran along the alley to the end, where he spotted the man walking towards a white house. "Stop! Stop Withhese words, he charged at the guy fearlessly and held fast to Kim. However hard the guy tried to get rid ofTJ. TJ wouldnt give inAt that very moment, Kim s parents arrived. Help! Help! "TJ shouted. Immediately Kim's father tookoff toward the guy. Seeing this. he guy hurriedly let go of Kim and broke into a run. Kim's father dashedup, caught hold of him and pressed him to the ground. Kim ran crying into her mother s arms. Minutes later,the police arrived and arrested the guy. TJ breathed a sigh of relief and smiled happily. T]'s quick thinkingand bravery saved a little girl in the neighborhood from unimaginable consequences

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