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平行四边形的识别

平行四边形的识别. 复习 : 1 、如图 ABCD 的对角线交于点 O , AC=24 , BD=38 , AD=28 ,则△ OBC 的周长 =____. A. D. O. B. C. C. F. D. A. E. B. 2 、在△ ABC 中, AB=AC , DE∥AC 交 AB 于 E , DF∥AB 交 AC 于 F ,则下列各式中不成立的是 ( ). A 、 DF=CF , DE=EB B 、 DF=AE , DE=AF C 、 DF-DE=DB D 、 DE+DF=AB.

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平行四边形的识别

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  1. 平行四边形的识别

  2. 复习:1、如图ABCD的对角线交于点O,AC=24,BD=38,AD=28,则△OBC的周长=____复习:1、如图ABCD的对角线交于点O,AC=24,BD=38,AD=28,则△OBC的周长=____ A D O B C

  3. C F D A E B 2、在△ABC中,AB=AC,DE∥AC交AB于E, DF∥AB交AC于F,则下列各式中不成立的是 ( ) A、DF=CF,DE=EB B、DF=AE,DE=AF C、DF-DE=DB D、DE+DF=AB

  4. 3、如图 ABCD中EF∥AD,GH∥AB,图中有______个平行四边形。其中面积相等的平行四边形是_______________ G A D E F M B C H

  5. F E D C 4、如图ABCD中AB比AD大2cm, ∠DAB的平分线AE交CD于E,∠ABC的平分线BF交CD于F,如果 ABCD的周长为24cm,则CE=_____cm;EF=______cm;FD=____cm A B

  6. 探索 在方格纸上画一个有一组对边平行且相等的四边形: 步骤1:画一线段AD 步骤2:平移线段AD到BC 步骤3:连结AB、DC,得到四边形ABCD, 其中, AD∥BC,AD=BC 另一组对边有什么关系?为什么? 结论:一组对边平行且相等的四边形是平行四边形

  7. 例1:如图,在▱ABCD中,已知点E和点F分别在AB和CD上,且BE=DF,连结DE和BF,试说明四边形BEDF是平行四边形。例1:如图,在▱ABCD中,已知点E和点F分别在AB和CD上,且BE=DF,连结DE和BF,试说明四边形BEDF是平行四边形。 F D C A E B G H 例2:如图,在▱ABCD中,已知点E和点F分别是AB和CD的中点,连结DE、BF、AF、CE,试说明四边形EGFH是平行四边形

  8. A D F O B C E 例3:如图△ABC中,E是BC的中点ED∥AC, DF∥BC,试说明DE、BF互相平分

  9. D A F E B C 练习1、如图四边形AEFD与四边形EBCF都是平行四边形试说明四边形 ABCD是平行四边形

  10. 思考题: ABC中,E、F分别是BC、CD上任意两点,且EF∥BD, 求证:S △ABE=S △ADF D A F B C E

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