Giải thích các bước giải:
a.Ta có: $Mx//Ob\to \widehat{xMO}+\widehat{aOb}=180^o$ (Hai góc trong cùng phía)
$\to\widehat{xMO}=180^o-\widehat{aOb}=50^o$
b.Ta có: $ON\perp Ob, Mx//Ob\to ON\perp Mx\to\widehat{ONM}=90^o$
Mà $\widehat{NMO}=\widehat{xMO}=50^o\to\widehat{NOM}=90^o-\widehat{NMO}=40^o$
c.Ta có: $\widehat{OMx'}=180^o-\widehat{NMO}=130^o=\widehat{aOb}$
Vì $My', Oy$ là phân giác $\widehat{OMx'},\widehat{aOb}$
$\to\widehat{aOy}=\dfrac12\widehat{aOb}=\dfrac12\widehat{OMx'}=\widehat{OMy'}$
$\to Oy//My'$
