2022英语周报八年级。44答案

【命题视角】本题考查等差与等比数列定义的判断,错位相减法求和,考查学生的逻辑推理能力和运算求解能力【解题分析】(1)由3an=2am1 b-1 3,3bn=am1 2b1 3,两式相减,得3(an-bn)=am1-b-1,an-b=3(a-1-b-1)(n≥2,n∈N),从而(an-b1}是以a1-b=1为首项,为公比的等比数列,a-b=()=同理由3an=2an1 b1 3,3b=a-1 2b.1 3,两式相加得3(an b)=3(a( b-) 6,(an b)=(a1 b=)王2(n>2,n∈N),从面{an b)是以a1 b=3为首项,2为公差的等差数列,a b=3 (x-1)×2÷2m 1.本)0(2)由()得c=a一层=()( b)=(2 1(3)-1,从雨=a a … G=3×1 5×号 7()2 … (2n-1(3)2 (2n 1)(33=3×3 5×(3)2 7×(3)2 … (2n-1)(-1 (2n 1)(3),所以T=3×1 2×()2 2×()2 … 2()1-(2n 1)(3)=42n ,T=6-57 63°的0<2,0=:0落(又T =6-35 3=6-一81,所以=2解得n=5木【价射服命

书面表达One possible version:NOTICEA sports meeting will be held in the playground of our school from next Thursday to FridayAs you know, the pressure of study is very heavy now, especially for those senior 3. So the purpose of the sports meeting is to letevery student get relaxed, as a result of which we students can live happily and heal thilyEveryone is welcome to take part in it. Those who perform excellently at the sports meeting will get prizes. But don' t take theresults so serously because taking part is more important than the result. Good luck to everyone!

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