高三北师大英语周报第22期答案

Possible version:Tom thought for a moment and was about to agree, but he changed his mind. " I can't, BenYou know Aunt Polly wants the fence to be perfect. It has to be done very carefully.I don'tthink you can do it well enough. "He said to Ben, who looked a little discouraged. "Oh, Tom, I'llbe careful. Please let me have a try I would like to give you my apple if you agree, "requestedBen In the end, Tom gave up the brush with unwillingness on his face, but joy in his heartWhile ben worked at the fence in the hot sun Tom sat under a tree, eating the apple, andlanning how to get more help. many of his other friends walked past him. Each one came topllaugh, but remained to whitewash the fence because they all got interested in it. One by one, theywhitewashed the fence, each giving Tom a reward, big or small. If he hadn't run out ofwhitewash, Tom would have owned everything belonging to his friends. He had a most wonderful andidle Saturday, sitting in the shade and enjoying the rewards that his friends gave him

12.(1)由椭圆的离心率为,得a2=2(a2-b2)又当y=1时因此椭圆C的方程为(2)设A(),B(联立得2得(2k2+1)x2+4kmx+2m2-4=0,由△>0得m2<4k2+2,(兴)4km2m且2k+1,因此yZkm所以D2k2+12k2+12km又N(0,-m),所以|ND|22k2+12k2+1整理得|ND/24m2(1+3k2+k4)(2k2+1)2因为|NF所以D4(k4+3k2+1)8k2+3NF|2(2k2+1)2(2k2+1)2令t=8k2+3,t≥3,故2k2+1所以ND/2NF2(1+1)2=1+-1616t+-+2令y=t+所以y=1-1当t≥3时,y>0从而y-t十在[3,十∞)上单调递增,因此1+等号当且仅当t=3时成立,此时k=0所以I NFI由(*)得-√2

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