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cosB=(a^2+c^2-b^2)/(2ac)=[a^2+c^2-(a+c)^2/4]/(2ac) =[(3/4)(a^2+c^2)-(1/2)ac]/(2ac)=(3/8)[(a/c)+(c/a)-(2/3)] ≥(3/8)(2-2/3)=1/2 所以:0
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