Junior high school mathematics: A and B are two fixed points on the same side of the straight line L, and the fixed-length segment PQ moves in parallel on L. When PQ moves to what position, what is the shortest length of AP+PQ+QB? Connect the CD with the symmetry point D of B about L, cross L to E, and move the B-side endpoint (such as Q) of PQ to E. At this time, AP+PQ+QB is the minimum.

(AP+PQ+QB = AC+CD。 If you put q in another position, there will always be.

AP+PQ+QB=AC+CQ+QD>AC+CD。 )