2022七年级下册英语周报第4答案
18.解:(1)证明:取EB的中点N,连接AN,GN,在△EBC中,NG∥BC且NG=BC,又AD∥BC且AD=BC,所以NG∥AD且NG=AD,所以四边形ADGN是平行四边形,从而DG∥AN(2分)又矩形ABEF和直角梯形ABCD所在的平面相互垂直且交于AB,EB⊥AB,所以EB⊥平面ABCD,同时CB⊥AB,所以CB⊥平画ABEF,所以CB⊥AN、CB⊥EF,因为DG∥AN,所以CB⊥DO,(3分)因为G,H分别为CE,CF的中点,所以G1∥EF,CB⊥GH,(4分)又DG∩GH=G,所以BC⊥平面DGH,(分(2)由(1)知以B为原点,BA,BC,BE所在直线分别为x;y,x轴建立空间直角坐标系(如图所示),NC因为DC=(-2,1,0),CF=(2,-2,2),还=(-2,2,0),由点M在棱FC上,设C=1CF=(2,-2,2),DM=DC+C商=(2-2,1-2,2),因为DM⊥AC,所以D·AC=-2(2-20)+2(12)+0=0,解得A=(7分)设平面MAD的法向量m=(x,y,a),由Tn⊥D(-x-1x+t=0取z=1,得n=(3,0,1)(9分)取平面ADF的法向量为吗=(1.0)(10分)设二面角M-AD=F的平面角为a,则有cos|cos
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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