1. (2010·希望杯竞赛) In right Fig.，if the length of the segment AB is 1，M is the midpoint of the segment AB，and point C divides the segment MB into two parts such that MC：CB=1：2，then the length of AC is 。

 (英汉词典：length 长度；segment 线段；midpoint 中点；divides…into 分为，分成)