问答题(2021年北京市

已知在△ABC中,c=2bcosB,C=2π/3.

(1)求B的大小;

(2)在三个条件中选择一个作为已知,使△ABC存在且唯一确定,并求BC边上的中线长度.①c=b;②周长为4+2;③面积为S△ABC=3/4.

答案解析

(1)由正弦定理b/sinB=c/sinC,得sinC=2sinBcosB=sin2B,故C=2B(舍)或C+2B=π.所以B=A=π/6.(2)由(1)知,c= b,故不能选①.选②,设BC=AC=2x,则AB=2 x,故周长为(4+2)x=4+2√3,解得x=1.从而BC=AC=2,AB=2,设BC的中点为D,则在△ABD中,由余弦定理,cosB=(AB2+BD2-AD2)/(2...

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