Ta có: $a // b (GT)$
$+) ⇒ \widehat{A1} = \widehat{B1}$ (hai góc so le trong)
mà $\widehat{A1} = 60^{o} ⇒ \widehat{B1} = 60^{o}$
$+) \widehat{A1} + \widehat{B2} = 180^{o}$ (hai góc trong cùng phía)
$⇒ 60^{o} + \widehat{B2} = 180^{o}$
$⇒ \widehat{B2} = 180^{o} - 60^{o} = 120^{o}$