Question

设A,B,C为△ABC的三个内角,向量m=（sinB+sinC,0),向量n=（0,sinA) 且｜向量m｜^2-｜向量

Answer 1

m^2-n^2=(sinB+sinC)^2-(sinA)^2=sinBsinC,①

由正弦定理,(b+c)^2-a^2=bc,

∴b^2+c^2-a^2=-bc,

1.cosA=-1/2,A=120°.

2|B-C|