20英语书周报答案

8.解:(1)若选①,由条件及正弦定理可知sinB=y33sin Asin Ct sin Ccos A(1分)即si(A+C= Yesin Asin c-+ sin Ccos a,(2分)/3所以 sin acos c+ cos Asin C= yesin asin o+ sin Ccos a,(3分)则 sin Acos C=3 Esin csin a,(4分)又sinA≠0,所以tanC=3(5分)因为C∈(0,π),所以C(6分)若选②,由条件可知 cOs Acos B+ sin asin B=2 sin Asin B、、7(1分)所以 cos Acos B- sin Asin B=-l则cos(A+B)即cosC(5分)因为C∈(0,x),所以C=x(6分)若选③由条件及正弦定理可知snC=2 2sin Bsin Csin a 2cos Acos B+1(1分因为sinC≠,所以smA=2A0B+1,(2分)整理得2 COs Acos b-2 sin asin B=-1,(3分)所以cos(A+B)=-1即cosC≤、l(5分)因为C∈(0,),所以C=(6分)(2)由余弦定理得7=a2+4-2a,解得a=3或a=-1(舍去)(8分因为BD=2DC,所以DC=1分)在△ACD中,AD2=22+12-2×2×1故AD=√3(12分)

One possible versionThe Most Beautiful Place in My HeartAs a 17-year-ol teenager, I ve been to many great cities, among which Luoyang is the most beautiful one inlocated on the north bank of the luo river in western Henan pruvinee l aovany is surrounded by mountains on threesidles, high in the west and low in the east. At the end of 2019, Luoyang had a population of more than 7. I millionI love Luoyang so much due to the fact that as one of the four ancient capitals of China, Luoyang has a civilizationhistory of 5, 000 years. Whats more, the number of its heritage sites is second only to Beijing in cities of China. And itfamous tourist attractions inelude Lungmen Grottoes, Baima Temple, Guanlin and so on

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