Giải thích các bước giải:

Ta có:

$\widehat{BOE}$

$=180^o-(\widehat{OEB}+\widehat{OBE})$

$=180^o-(\widehat{AEO}+\widehat{AEB}+\widehat{ABO}+\widehat{ABE})$

$=(180^o-\widehat{AEB}-\widehat{ABE})-\widehat{OEA}-\widehat{OBA}$

$=\widehat{EAB}-\dfrac12\widehat{DAE}-\dfrac12\widehat{ABC}$

$=\dfrac12(\widehat{EAB}-\widehat{DAE})+\dfrac12(\widehat{EAB}-\widehat{ABC})$

$=\dfrac12\widehat{ADE} +\dfrac12\widehat{ACB}$

$=\dfrac12\widehat{ABC}+\dfrac12\widehat{ACB}$ vì $DE//CB$

$=\dfrac12(\widehat{ABC}+\widehat{ACB})$