$a//b$
$→\widehat{B_1}=\widehat{A_1}=55°$ ( Hai góc đồng vị )
Mà $\widehat{A_2}+\widehat{A_1}=180°$ ( Hai góc kề bù )
$⇔\widehat{A_2}+55°=180°$
$⇔\widehat{A_2}=125°$
$c//d$
$→\widehat{D_2}=\widehat{A_2}=125°$ ( Hai góc đồng vị )
$a//b$
$→\widehat{C_2}=\widehat{D_2}=125°$ ( Hai góc đồng vị )
Mà $\widehat{C_1}+\widehat{C_2}=180°$ ( Hai góc kề bù )
$⇔\widehat{C_1}+125°=180°$
$⇔\widehat{C_1}=55°$
Vậy $\widehat{C_1}=55°$ và $\widehat{D_2}=125°$
