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depvazoi


untitlTRJHTHJed.GIF

Vẽ $GI//AD; MF//CD (I \in CD ; M \in AD)$
Gọi $\left\{H\right\} = EF \cap GK ; \left\{N\right\} = GK \cap AD$
$=> MF \perp AD; GI \perp CD$ và $\widehat{HGI}=\widehat{HNE} (slt)$
Mà $\widehat{ENH}=\widehat{EFM}$ (cùng phụ $\widehat{NEH}$)
$=> \widehat{HGI}=\widehat{EFM}$
Mà $GI=MF (=AD)$
$=> \Delta MEF = \Delta IKG (cgv-gn kề)$
$=> EF=GK$
 
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