Giải thích các bước giải:
Ta có: $Ax//By$
$\to180^o-\widehat{yBA}=\widehat{xAB}$
Ta có:
$\widehat{yBC}=360^o-\widehat{yBA}-\widehat{ABC}$
$\to \widehat{yBC}=(180^o-\widehat{yBA})+(180^o-\widehat{ABC})$
$\to\widehat{yBC}=\widehat{xAB}+(\widehat{BAC}+\widehat{BCA})$
$\to\widehat{yBC}=\widehat{xAB}+\widehat{BAC}+\widehat{BCA}$
$\to\widehat{yBC}=(\widehat{xAB}+\widehat{BAC})+\widehat{ACB}$
$\to\widehat{yBC}=\widehat{xAC}+\widehat{ACB}$
$\to đpcm$