1　问题的提出习题　已知圆台的母线长为 2 ,上、下底面半径分别为 1和 2 ,有一动点 P从下底面圆周上一点 A开始出发 ,绕圆台侧面一周再回到 A点 ,求动点 P经过的最短路程 .为了便于研究 ,把问题一般化 :“已知圆台的母线长为 l,上、下底面半径分别为 r′和r,有一动点 P从下底 1 The question is presented. The length of the generatrix is ​​known as 2, the radius of the upper and lower bases are 1 and 2, respectively. There is a starting point P starting from point A on the circumference of the lower base, and then returning to the point A around the side of the circular table. The shortest distance traveled by the point P. To facilitate the study, generalize the problem: “The bus of the known circular table is l, the upper and lower bottom radii are r′ and r respectively, and there is a moving point P from the bottom.