a+c=2b
(a+c)^2=(2b)^2
4b^2=a^2+2ac+c^2
根据余弦定理:
b^2=a^2+c^2-2accosB
cosB=(a^2+c^2-b^2)/(2ac)=(3a^2+3c^2-2ac)/(8ac)
=(3/8)*(a^2+c^2)/(ac)-1/4>=(3/8)*(2ac)/(ac)-1/4=1/2
B
a+c=2b
(a+c)^2=(2b)^2
4b^2=a^2+2ac+c^2
根据余弦定理:
b^2=a^2+c^2-2accosB
cosB=(a^2+c^2-b^2)/(2ac)=(3a^2+3c^2-2ac)/(8ac)
=(3/8)*(a^2+c^2)/(ac)-1/4>=(3/8)*(2ac)/(ac)-1/4=1/2
B