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2006年全国高中数学联赛试题:已知△ABC,若对任意t∈R,(?)≥(?),则△ABC一定为( ).(A)锐角三角形(B)钝角三角形
The National High School Mathematics League 2006 test: Known △ABC, if for any t ∈ R, (?) ≥ (?), △ ABC must be (). (A) acute triangle (B) obtuse triangle