m,n向量共线
2sinBcosB=√3cos2B
sin2B-√3cos2B=0
2sin(2B-π/3)=0
2B-π/3=0,2B-π/3=π(舍)
B=π/6
b^2=a^2+c^2-2accosB
4-2√3=a^2+c^2-√3ac>=2ac-√3ac
ac
m,n向量共线
2sinBcosB=√3cos2B
sin2B-√3cos2B=0
2sin(2B-π/3)=0
2B-π/3=0,2B-π/3=π(舍)
B=π/6
b^2=a^2+c^2-2accosB
4-2√3=a^2+c^2-√3ac>=2ac-√3ac
ac