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阅读理 课外兴趣小组活动时，老师提出了如下问题： 如图1，△ABC中，若AB=5，AC=3，求BC边上的中线AD的取值范围． 小明在组内经过合作交流，得到了如下的解决方法：延长AD到E，使得DE=AD，再连接BE（或将△ACD绕点D逆时针旋转180°得到△EBD），把AB、AC、2AD集中在△ABE中，利用三角形的三边关系可得2＜AE＜8，则1＜AD＜4． 感悟：解题时，条件中若出现“中点”“中线”字样，可以考虑构造以中点为对称中心的中心对称图形，把分散的已知条件和所求证的结论集中到同一个三角形中． （1）问题解决： 受到（1）的启发，请你证明下面命题：如图2，在△ABC中，D是BC边上的中点，DE⊥DF，DE交AB于点E，DF交AC于点F，连接EF． ①求证：BE+CF＞EF； ②若∠A=90°，探索线段BE、CF、EF之间的等量关系，并加以证明； （2）问题拓展： 如图3，在四边形ABDC中，∠B+∠C=180°，DB=DC，∠BDC=120°，以D为顶点作一个60°角，角的两边分别交AB、AC于E、F两点，连接EF，探索线段BE、CF、EF之间的数量关系，并加以证明．
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【单选题】回溯法求解0-1背包问题时,第i层的左儿子是否可行所用约束函数为( )。
【单选题】SECTION A CONVERSATIONS Directions: In this section you will hear several conversations. Listen to the conversations carefully and then answer the questions that follow. 听力原文：M: One thing you listener...
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quite slim
B.
on diet
C.
extremely overweight
D.
doing exercises to lose weight
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A.
a bookstore
B.
the telephone company
C.
a map of the town
D.
a shoe repair shop
【判断题】You will always hear 2 speakers in Section 1, and they will have an everyday conversation.
A.
正确
B.
错误
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A.
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B.
It is unusual for the students not to do her homework.
C.
The student gets to do her homework all the time.
D.
The student has a lot of homework to make up.
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【单选题】回溯法求解0-1背包问题时,第i层的左儿子是否可行所用约束函数为( )。