A boat crosses the river, starting from point A. If it keeps a course perpendicular to the shore, then after time t1 after departure it ends up at point C, which lies at a distance S below point B (Fig.a). If it holds a course at the same speed at a certain angle a to straight line AB, then after time t2 the boat will end up at point B (Fig.b). Determine: the width of the river I, the speed of the boat u, the speed of the river v and the angle a.

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