作者Zyca (swun mitru valy)
标题[转录]◇ IMO 竞赛第五题
时间Wed Jan 5 20:25:30 2000
Let I be the incentre（内心）of triangle ABC. Let the incircle（内接圆）
of ABC touch the sides BC,CA and AB at K,L and M,respectively.The line
through B parallel to MK meets the line LM and LK at R and S,respectively.
Prove that ∠RIS is acute.（锐角）
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