a,b,c成等比数列,∴a/b = b/c,b^2=ac根据余弦定理,b^2=a^2+c^2-2accosBcosB = (a^2+c^2-b^2)/(2ac) = (a^2+c^2-ac)/(2ac) = (a^2+c^2)/(2ac) -1/2 ≥ 1-1/2 = 1/2cosB≥1/20 ＜ B ≤ 60°b^2 = a^2+c^2-2accosB ≤ a...