`15)`

GT | `AB` `_|_` `EF` 

       `CD` `_|_` `EF`

KL | `AB` `////` `CD`

`+)` Ta có: `AB` `_|_` `EF` `=>` `\hat{EHB}` `=` `90^@`

`CD` `_|_` `EF` `=>` `\hat{HGD}` `=` `90^@`

Mà 2 góc này ở vị trí đồng vị 

`=>` `AB` `////` `CD`

`16)` 

`+)` Ta có: `\hat{ACx}` và `\hat{ACB}` là 2 góc kề bù

`=>` `\hat{ACB}` `=` `180^@` `-` `\hat{ACx}`

`=>` `\hat{ACB}` `=` `180^@` `-` `100^@`

`=>` `\hat{ACB}` `=` `80^@`

`+)` `\triangle ABC` có: `\hat{BAC}` `+` `\hat{ABC}` `+` `\hat{ACB}` `=` `180^@`

`=>` `\hat{BAC}` `=` `180^@` `-` `\hat{ABC}` `-` `\hat{ACB}`

`=>` `\hat{BAC}` `=` `180^@` `-` `55^@` `-` `80^@`

`=>` `\hat{BAC}` `=` `45^@`