余弦定理：b^2=a^2+c^2-2ac cosB则 a^2+c^2-b^2=2ac cosB=accosB=1/2角B=60度
b^2=a^2+c^2-2ac cosB=a^2+9a^2-2*a*3a *1/2=10a^2-3a^2 =7a^2 b=√7a