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# 梯形中常见辅助线 - PowerPoint PPT Presentation

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Presentation Transcript

1根据转化思想，梯形的问题应该转化成什么图形的问题去解决？

2梯形常用的辅助线有哪些？

E

D

A

C

B

50°

5

50°

5

80°

50°

9

D

A

F

C

B

6

7

7

6

2

E

8

= 20cm

M

N

E

D

A

F

E

B

C

5

5

5

11

E

F

(1)延长DE与CB相交于点F 证△AED与△BEF 全等

(2)将△AED绕点E旋转180°，到△BEF的位置，

S梯形ABCD＝S△DCF=2倍S△DCE

（1）证明：延长DE与CB相交于点F

（2）证明：将△AED绕点E旋转180°，到△BEF的位置

G

A

D

F

E

B

C

1、把上下底之和，两对角线转移到同一个三角形BDE中

2、△ABD与△CDE面积相等

S梯形ABCD＝S△BDE

3、 BD⊥AC推出BD⊥DE得到直角三角形BDE

1

4

3

4

1

4

F

5

F

12

20

15

20

E

M

=150