问题详情：
如图1，线段AC＝6cm，线段BC＝15cm，点M是AC的中点，在CB上取一点N，使得CN：NB＝1：2，求MN的长．
【回答】
解：∵M是AC的中点，AC＝6，
∴MC＝
AC＝6×
＝3，·············································································· 1分
又因为CN∶NB＝1∶2，BC＝15，
∴CN＝15×
＝5，·························································································· 3分
∴MN＝MC＋CN＝3＋5＝8，
∴MN的长为8 cm·························································································· 4分
知识点：直*、*线、线段
题型：解答题