Kẻ Oz song song đường thẳng b
mà $\widehat{zOc}$ $SLT$ $\widehat{bCO}$
$⇒\widehat{zOc}=\widehat{bCO}=48^o$
$\widehat{zOc}+\widehat{zOA}=\widehat{AOc}$
$⇒48^o+\widehat{zOA}=90^o$
$⇒\widehat{zOA}=90^o-48^o=42^o$
$\widehat{aAO}=\widehat{zOA}(=42^o)$
mà $\widehat{aAO} SLT \widehat{zOA}$
$⇒a//Oz$
mà $b//Oz$
$⇒a//b$