2021-2022 英语周报 七年级 csx 10答案

18.(1)因为An=n2,所以An-1=(n-1)2,n≥2,所以又a1=1也适合上式,所以an=2n-1,n∈N故bn+1-bn)=1所以数列{bn}是以2为首项,1为公差的等差数列,所以Bn=n·2+(2)因为a=Bn,所以an+1=Bn+1所以an+1-an=BB又an+1-an=2(bn+1-b),所以Bn+1-Bn=2(bn+1-bn),即bn+1=2(bn+1-bn),即所以数列{bn}是以1为首项,2为公比的等比数列,所以bn=2-1,所以an=B所以数列{bn}是以1为首项,2为公比的等比数列,所以bn=2-1,所以an=B所以又因为(2n-1)·(2b所以Sn

reading, with a confused look on her face. Dannis stoodup,glared at her and shouted, "You stole my wallet! "Thecrowds began to gossip. The girl turned deadly pale, loo-king so restless that words failed her. A strange silencehung in the air. Abruptly, she leaped from the seatParagraph 2The gird ran and Dannis ran after her. After a whileDannis finally grabbed her by the arm. At exactly thatmoment,a policeman approached them and asked aboutwhat happened. Dannis hastened to explain it while the girlfrowned with a shake of her head. After further enquiriesthe policeman took out a wallet and handed it to Dannisadding that someone had found it in the toilet. What a mis-take he had made! Overwhelmed by guilt, Dannis made anapology to the girl第三节One possible versionParagraph 1A seat beside her was empty, and Dannis occupied itHe felt a wave of anger swept over him, "Where is my wal-let? he yelled. The black-haired girl glanced up from her

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