Question

在△ABC中,abc分别是角ABC的对边,A=2B,cosB=根号6/3,求c/b的值

快，谢谢

Answer 1

解由正弦定理得
c/b
=sinC/sinB

=sin(π-A-B)/sinB
=sin(π-3B)/sinB
=sin3B/sinB
=【3sinB-4sin^3B】/sinB【三倍角公式sin3a=3sina-4(sina)^3】
=3-4sin^2B
=4-sin^2B-1
=4cos^2B-1
=5/3
答题不易，且回且珍惜
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