题目内容
(12分) 如图，在△ABC中，ME和NF分别垂直平分AB和AC．
(1) 若BC =" 10" cm，试求△AMN的周长．
(2) 在△ABC中，AB = AC，∠BAC = 100°，求∠MAN的度数．
(3) 在 (2) 中，若无AB = AC的条件，你还能求出∠MAN的度数吗？若能，请求出；若不能，请说明理由．
（1）
cm
（2） 
（3）能，证明略。
解析解：(1) ∵ME垂直平分AB
∴MA = MB·············································································· 1分
∵NF垂直平分AC
∴NA = NC··············································································· 2分
∴
cm······················· 4分
(2) ∵AB = AC，
∴
······································································· 4分
∵MA = MB
∴
··································································· 5分
∵NA = NC
∴
··································································· 6分
∴·········································· 8分
(3) 能．理由如下：·········································································· 9分
∵MA = MB
∴∠MAB =∠B
∵NA = NB
∴∠NAC =∠C
∴
·································· 12分
BD＝AC，求∠B︰∠C 的比值
），已知抛物线y=ax2+bx+c（a≠0）经过三点A、B、O（O为原点）．
cm/s的速度运动，连接BQ、PQ，设△BQP面积为S（cm2），点P、Q运动的时间为t（s），求S与t的函数关系式，并写出t的取值范围；
）