Ta có: \(\widehat{A}+\widehat{D}=180^o\) (trong cùng phía) (1)
\(\widehat{A}-\widehat{D}=20^o\Rightarrow\widehat{A}=20^o+\widehat{D}\left(2\right)\)
Thay (2) vào (1): \(20^o+2\widehat{D}=180^o\)
\(\Rightarrow\widehat{D}=80^o\)
\(\Rightarrow\widehat{A}=20^o+80^o=100^o\)
Lại có: \(\widehat{B}+\widehat{C}=180^o\) (trong cùng phía)
\(\Rightarrow2\widehat{C}+\widehat{C}=180^o\)
\(\Rightarrow\widehat{C}=60^o\)
Khi đó: \(\widehat{B}=180^o-60^o=120^o\).