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17.参考答案解:[文](1)因为cos2[章]B=cos(A+C[来]),所以2cos2[自]B-1=-cosB[知],(2分)解得co[嘛]sB=或cosB=[答]-1(舍).(4分[案])由B为三角形的内[网]角,得B=-.(5[文]分)3(2)因为 asin A+ csin c=6sinB,所[章]以由正弦定理,得a[来]2+c2=6b=1[自]8.(6分)12+[知]c2-9=9,所以[嘛]a=9.(8分)因[答]为cosB=2=-[案]2ac2ac因为([网]a+c)2=a2+[文]c2+2ac=18[章]+18=36,所以[来]a+c=6,(9分[自])所以△ABC的周[知]长a+b+c=9.[嘛](10分)
书面表达One possible version:N[答]OTICEA sports meeting will be held in the playgroun[案]d of our school from next Thursday to FridayAs you know, the pressure of study is very heavy now, especiall[网]y 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 thilyEver[文]yone is welcome to take part in it. Those who perform excellent[章]ly at the sports meeting will get prizes. But don' t take theresult[来]s so serously because taking part is more important[自] than the result. Good luck to everyone![知]
