sin^2 A=sin^2 B+sinBsinC+sin^2 C 求A角

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sin^2 A=sin^2 B+sinBsinC+sin^2 C 求A角
sin^2 A=sin^2 B+sinBsinC+sin^2 C 求A角

sin^2 A=sin^2 B+sinBsinC+sin^2 C 求A角
a/sinA=b/sinB=c/sinC
sin^2 A=sin^2 B+sinBsinC+sin^2 C
所以a^2=b^2+bc+c^2
b^2+c^2-a^2=-bc
cosA=(b^2+c^2-a^2)/(2bc)=-1/2
所以A为120度

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