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minimum distance of 2 ships, a navy one and an enemy one

Source: Vietnamese MO (VMO) 1965

August 22, 2018
geometryminimum

Problem Statement

At a time t=0t = 0, a navy ship is at a point OO, while an enemy ship is at a point AA cruising with speed vv perpendicular to OA=aOA = a. The speed and direction of the enemy ship do not change. The strategy of the navy ship is to travel with constant speed uu at a angle 0<ϕ<π/20 < \phi < \pi /2 to the line OAOA. 1) Let ϕ\phi be chosen. What is the minimum distance between the two ships? Under what conditions will the distance vanish? 2) If the distance does not vanish, what is the choice of ϕ\phi to minimize the distance? What are directions of the two ships when their distance is minimum?