`text{+) Ta có: a // b (gt)}`
`text{⇒}` $\widehat{A1}$ `text{=}` $\widehat{B2}$ `text{(đồng vị)}`
`text{⇒}` $\widehat{B2}$ `text{=}` $90^o$
`text{⇒ x =}` $90^o$
`text{+) Ta có: a // b (gt)}`
`text{⇒}` $\widehat{C3}$ `text{+}` $\widehat{D3}$ `text{=}` $180^o$ `text{(trong cùng phía)}`
`text{⇒}` $130^o$ `text{+}` $\widehat{D3}$ `text{=}` $180^o$
`text{⇒}` $\widehat{D3}$ `text{=}` $180^o$ - $130^o$
`text{⇒}` $\widehat{D3}$ `text{=}` $50^o$
`text{⇒ y =}` $50^o$
