$$\eqalign{
& a)\,\,Ta\,\,co: \cr
& \widehat {CBy} + \widehat {BCz} = {180^0} - {b^0} + {b^0} = {180^0} \cr
& \Rightarrow Day\,\,la\,\,2\,\,goc\,\,bu\,\,nhau \cr
& Ma\,\,hai\,\,goc\,\,nay\,\,nam\,\,o\,\,vi\,\,tri\,\,trong\,\,cung\,\,phia. \cr
& \Rightarrow By//Cz \cr
& b)\,\,\widehat {ABy} = {360^0} - \widehat {ABC} - \widehat {CBy} \cr
& = {360^0} - \left( {{a^0} + {b^0}} \right) - \left( {{{180}^0} - {b^0}} \right) \cr
& = {360^0} - {a^0} - {b^0} - {180^0} + {b^0} \cr
& = {180^0} - {a^0} \cr
& \Rightarrow \widehat {ABy} + \widehat {xAB} = {180^0} - {a^0} + {a^0} = {180^0} \cr
& \Rightarrow Day\,\,la\,\,2\,\,goc\,\,bu\,\,nhau \cr
& Ma\,\,hai\,\,goc\,\,nay\,\,nam\,\,o\,\,vi\,\,tri\,\,trong\,\,cung\,\,phia. \cr
& \Rightarrow Ax//By \cr} $$
