20182022英语周报新目标答案

【解析】1)设AC的中点为O,连接BO,P(2由题意,得PA=PB=PC=√2,=《PO=1,AO- BO=CO=1.2分因为在△PAC中,PA=PC,O为AC的中点,所以PO⊥AC,………a-138+x284(+1)因为在△POB中,PO=1,OB=1,PB=√2,4分PO+OB=PF,所以 POL因为AC∩OB=O,AC,OBC平面ABC,所以PO⊥平面ABC因为PC平面PAC:所以平面PAC⊥平西ABC.……((2)由(1)知,BO⊥PO,BO⊥AC,BO⊥平面PAC,所以∠BMO是直线BM与平面PAC所成的角,BO且tanBMO-OMOM所以当OM最短时,即M是PA的中点时,∠BM最大,7分由PO⊥平面ABC,OB⊥AC,所以PO⊥OB,PO⊥OC,于是法OC,OBOP所在直线分别为x轴,y轴,轴建立如图所示空间直角坐标系,O0.0C0BC01.0.AC-1.0,P(0D.(-+,0)9分成=(1.-1,0,)(1,.-1,威=(-)设平面MBC的法向量为m=(,y,)令21=1,得y2=1,的=3,即m=(1,1,3).……………………………““”““……BCE=0得10分MC=0,(3x1-1=0,设平面PBC的法向量为n=(x,y,),0,-32=0令x=1,得3=1,a=1,即n=(1,1,1).……………………………由得=011P=0/33cos(n, m)m:1方312分由图可知,二面角P-BC一M的余弦值为5√3点P为圆C1上的任意一点,线

第一节Dear Mr GeorgeI am Li Hua, a volunteer of the activity"Englishi Communication to the Mountains". Now, I'm writing toexpress my sincere apology to you for not being present atthe activity to be held next Monday.The reason for my absence is that I suddenly fell illMy parents sent me to hospital, and I have to stay therefor a week. As for my volunteer work, I have asked myclassmate Tom to do it.Sincerely hoping that you can accept my apologiesYours.Li Hua

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