Giải thích các bước giải:

a.Xét $\Delta AOD, \Delta BCO$ có:

$\widehat{AOD}=\widehat{BOC}$

$\widehat{ADO}=\widehat{BCO}$ vì $\widehat{ADB}=\widehat{ACB}$

$\to \Delta AOD\sim\Delta BOC(c.g.c)$

b.Từ a $\to \dfrac{OA}{OB}=\dfrac{OD}{OC}$

Mà $\widehat{AOB}=\widehat{COD}$

$\to \Delta AOB\sim\Delta DOC(c.g.c)$

c.Từ b $\to \widehat{OBA}=\widehat{OCD}$

$\to \widehat{EBD}=\widehat{ACE}$

Mà $\widehat{BED}=\widehat{AEC}$

$\to \Delta EDB\sim\Delta EAC(g.g)$

$\to \dfrac{ED}{EA}=\dfrac{EB}{EC}$

$\to EA.EB=EC.ED$