（a+b+c)(b+c-a)=3bc,sinA=?

zshzl 1年前 已收到2个回答 举报
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laylyl 幼苗

共回答了27个问题采纳率：88.9% 举报

(a+b+c)(b+c-a)=3bc
(b+c)^2-a^2=3bc
b^2+2bc+c^2-3bc=a^2　即：a^2=b^2-bc+c^2·····················1
根据余弦定理得：
a^2=b^2+c^2-2bcCOSA　···················································2
联立1、2解得：COSA=1/2
所以,SinA=√3/2

1年前

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lili9494 幼苗

共回答了3个问题 举报

(a+b+c)(b+c-a)=3bc
(b+c)^2-a^2=3bc
b^2+2bc+c^2-3bc=a^2　即：a^2=b^2-bc+c^2·····················1
根据余弦定理得：
a^2=b^2+c^2-2bcCOSA　···················································2
联立1、...

1年前

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