衡水金卷先享题电子版 答案

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17.参考答案解:(1)因为cos2B=cos(A C),所以2cos2B-1=-cosB,(2分)解得cosB=或cosB=-1(舍).(4分)由B为三角形的内角,得B=-.(5分)3(2)因为 asin A csin c=6sinB,所以由正弦定理,得a2 c2=6b=18.(6分)12 c2-9=9,所以a=9.(8分)因为cosB=2=-2ac2ac因为(a c)2=a2 c2 2ac=18 18=36,所以a c=6,(9分)所以△ABC的周长a b c=9.(10分)

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