My husband and I were paying a visit 61 my parents in Tucson. We went to a fast-food restaurant for dinner together. My husband went to the counter 62 (order )dishes and I stood with my parents. My dad is 90 years old. He can hardly see very well and walks with a stick. I was scanning the restaurant, 63 (wait) to sit at the first table that was 64 (convenient)than others. A woman who was sitting with her son made eye contact with me and asked me to come to her. With a 65 (puzzle) look on my face, I pointed to myself and said , “Me ?如图,点C在线段AB上,点M、N分别是AC、BC的中点.(1)若AC=8cm,CB=6cm,求线段MN的长;(2)若C为线段AB上任一点,满足AC+CB=a,其它条件不变,你能猜想MN的长度吗?写出你的结论并说明理由;(3)若点C在线段AB的延长线上,且满足AC-BC=b,M、N分别为AC、BC的中点,你能猜想MN的长度吗?请画出图形并写出你的结论(不必说明理由).