IMO 1983 LL BUL14

Let l be tangent to the circle k at B. Let A be a point on k

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IMO 1983 LL BUL14

Origin: BUL

Problem

Let l be tangent to the circle k at B. Let A be a point on k and P the foot of perpendicular from A to l. Let M be symmetric to P with respect to AB. Find the set of all such points M.