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1、 如图,AB∥DC,AE⊥BD,CF⊥BD,BF=DE.求证:△ABE≌△CDF.证明:∵AB∥DC,∴∠1= ∠2.∵AE⊥BD,CF⊥BD,∴∠AEB=∠CFD=90°.∵BF=DE,那么BE+EF=DF+EF∴BE= DF. 在△ABE和△CDF中 ∠1=∠2,∠AEB=∠CFD,BE=DF∴△ABE≌△CDF(AAS). 2、如图,OA⊥AC,OB⊥BC,填空:(1)利用“角的平分线上的点到角的两边的距离相等”,已知∠1 =∠2,可得AC=BC;(2)利用“角的内部到角两边距离相等的点在角的平分线上”,已知AC = BC,可得∠1=∠2;
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3、如图,CD=CA,∠1=∠2,EC=BC.求证:DE=AB.
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∵∠1=∠2那么∠1+∠ACE=∠ACE+∠2即∠BCA=∠ECD∵CD=CA,EC=BC∴△ABC≌△DEC(SAS)∴DE=AB4、如图,AB=DE,AC=DF,BE=CF.求证:AB∥DE.
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∵BE=CF,那么BE+EC=EC+CF即BC=EF∵AB=DE,AC=DF∴△ABC≌△DEF(SSS)∴∠ABC=∠DEF∴AB∥DE(同位角相等,两直线平行)5、如图,在△ABC中,D是BC的中点,DE⊥AB,DF⊥AC,BE=CF.求证:AD是△ABC的角平分线.
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∵D是BC中点,那么BD=CD∵DE⊥AB,DF⊥AC,那么∠BED=∠CFD=90°即△BED、△CFD是直角三角形BE=CF∴RT△BED≌RT△CFD(HL)∴∠B=∠C,那么AB=AC∴AB-BE=AC-CF即AE=AF∵AD=AD∴RT△ADE≌RT△ADF(HL)∴∠EAD=∠FAD即∠BAD=∠CAD∴AD平分∠BAC