1
m=(c,b),n=(sin2B,sinC),m与n垂直,则:m·n=(c,b)·(sin2B,sinC)=csin2B+bsinC=0
而由正弦定理:b/sinB=c/sinC,即:sinC=csinB/b,故:sin2B=-sinB,即:cosB=-1/2
故:B=2π/3
2
△ABC的面积:S△ABC=(1/2)acsinB=sqrt(3)ac/4=3sqrt(3)/4,故:ac=3,由余弦定理:
b^2=a^2+c^2-2accosB=b^2+c^2+3≥2bc+3=6+3=9,故b的最小值:3