题目

A Hole in the BoatOnce a boat owner decided to get his boat painted. So one day, he called a painter and asked him to paint his boat.The painter brought paint and brushes with him and began to paint the boat with bright red color as the owner asked him.While painting the boat, he realized that there was a hole in the hull( 船体 ) and decided to repair it. After he finished painting, the painter received his money from the owner and left.The next day, the owner of the boat came to the painter and presented him with a check, much higher than the payment for painting. The painter was surprised 如图所示，沿长方体表面系一根绳子连接A点与B点．如果让绳子的长度达到最短，其长度应是多少厘米？