IMO Shortlist 1998 problem G3

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Let I be the incenter of triangle ABC. Let K,L and M be the points of tangency of the incircle of ABC with AB,BC and CA, respectively. The line t passes through B and is parallel to KL. The lines MK and ML intersect t at the points R and S. Prove that \angle RIS is acute.
Izvor: Međunarodna matematička olimpijada, shortlist 1998