`a)` Ta có: `\hat{BAE}` `=` `\hat{AEF}` `=` `30^@` (2 góc so le trong)

`=>` `DA` `////` `EF`

`b)` Lại có: `DA` `////` `BC`

                 `DA` `////` `EF`

`=>` `BC` `////` `EF

Vì `BC` `////` `EF` (cmt)

`=>` `\hat{ABC}` `=` `\hat{AEF}` `=` `30^@` (2 góc đồng vị)

Vì `BC` `////` `EF` (cmt)

`=>` `\hat{AFE}` `=` `\hat{FCB}` `=` `60^@` (2 góc đồng vị)

`c)` `\triangle AEF` có: `\hat{AEF}` `+` `\hat{AFE}` `+` `\hat{EAF}` `=` `180^@`

`=>` `\hat{EAF}` `=` `180^@` `-` `\hat{AFE}` `-` `\hat{AEF}`

`=>` `\hat{EAF}` `=` `180^@` `-` `60^@` `-` `30^@`

`=>` `\hat{EAF}` `=` `90^@`

 `=>` `AB` `_|_` `AC` (đpcm)