CM: $\Delta AEI=\Delta BAH(Ch-Gn) \Longrightarrow EI=AH$
$\Delta AGJ=\Delta CAH(Ch-Gn) \Longrightarrow GJ=AH$
$\Longrightarrow EI=KG(=AH)$
CM: $\Delta IEK=\Delta JGK (Gn-Cv) \Longrightarrow EK=GK$
Lại có: $KL=AK \Longrightarrow AELG$ là hình bình hành $\Longrightarrow AE=LG; AE//LG \Longrightarrow \widehat{EAG}+ \widehat{AGL}=180^o$ (kề bù)
Mặt khác, ta có: $\widehat{BAC}+\widehat{EAG}=180^o$
$\Longrightarrow \widehat{AGL}=\widehat{BAC}$
Xét $\Delta AGL$ và $\Delta CAB$ có:
$ \widehat{AGL}=\widehat{BAC}$
$AG=AC; GL=AB(=AE)$
$\Longrightarrow \Delta AGL= \Delta CAB(c-g-c) \Longrightarrow AL=BC$