19．(10分)(1)请仔细观察图形(阴影部分)，指出所给虚线中哪些是图形的对称轴？

(2)下列图形是轴对称图形吗？如果是，分别画出它们的对称轴.

18．································································· 如图，在△ABC中，点O是∠ABC的平分线与线段BC的垂直平分线的交点，则下列结论不一定成立的是······ ( 　　)．

　　 (A)OB＝OC　　　　　　　　 (B)OD＝OF

　　 (C)OA＝OB＝OC　　　　 (D)BD＝DC

17．································································· 如图，等边三角形ABC中，AD是BC上的高，取AC的中点E，连结DE，则图中与DE相等的线段有········· ( 　　)．

　　 (A)1条　　　　　　　　　　　 (B)2条

　　 (C)3条　　　　　　　　　　　 (D)4条

16．······························································································ 已知等腰三角形的周长为24，腰长为x，则x的取值范围是·································································· ( 　　)．

　　 (A)x＞12　　　　　 (B)x＜6　　　　　　 (C)6＜x＜12　　　 (D)0＜x＜12

15．······························································································ 已知等腰三角形的一边等于3，一边等于6，那么它的周长等于······························································ ( 　　)．

　　 (A)12　　　　　　　 (B)12或15　　　　 (C)15　　　　　　　 (D)15或18

14．······························································································ 已知△ABC的周长为24，AB＝AC，AD⊥BC于D，若△ABD的周长为20，则AD的长为························· ( 　　)．

　　 (A)6　　　　　　　　 (B)8　　　　　　　　 (C)10　　　　　　　 (D)12

13．······························································································ 正五角星的对称轴有 ( 　　)．

　　 (A)1条　　　　　　 (B)2条　　　　　　　 (C)5条　　　　　　 (D)10条

12．······························································································ 下列图形不一定是轴对称图形的是······························································································ ( 　　)．

　　 (A)线段　　　　　　 (B)正方形　　　　　 (C)半圆　　　　　　 (D)三角形

11．······························································································ 下列是我国四大银行的商标，其中不是轴对称图形的是······································································ ( 　　)．

　　 (A)　　　　　　　　　 (B)　　　　　　　　　 (C)　　　　　　　　　 (D)

10．如图，在等腰梯形ABCD中，对角线AC与BD交于点O，图中全等的三角形是　　 　　　　　　　　　　　　　　　　　　．