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Given a straight line L , and two points P and Q lying on the same side of L . Mark a point T on L so that the sum of the lengths of PT and QT is minimal. (Hint: Consider the reflection image of P about the line L ) 已知一直線 L ,及兩點 P、Q 位於 L 的同一方。試在 L 上作一點 T 使得 PT 及 QT 的長度之和最小。(提示:可考慮 P 點於直線 L 上作反射的影像)