Vì \(a//b\left(gt\right)\) nên:
\(\Rightarrow\widehat{A_1}=\widehat{B_1}=80^0\left(đồng-vị\right)\)
Lại có: \(\widehat{B_2}+\widehat{B_1}=180^0\left(kề-bù\right)\)
\(\Rightarrow\widehat{B_2}=180^{0-}\widehat{B_1}=180^0-80^0\)
\(\Rightarrow\widehat{B_2}=100^0\)