Vì AB//CD nên [tex]\dpi{100} \widehat{A}+\widehat{D}=\widehat{B}+\widehat{C}=180^o[/tex](các cặp góc nằm ở vị trí hai góc trong cùng phía)
[tex]\dpi{100} \widehat{A}-\widehat{D}=20^o\Rightarrow \widehat{A}=\widehat{D}+20^o\\ \widehat{A}+\widehat{D}=\widehat{D}+20^o+\widehat{D}=2\widehat{D}+20^o=180^o\\ 2\widehat{D}=160^o\\\widehat{D}=80^o\Rightarrow \widehat{A}=\widehat{D}+20^o=80^o+20^o=100^o\\\widehat{B}=2\widehat{C}\ \widehat{B}+\widehat{C}=2\widehat{C}+\widehat{C}=3\widehat{C}=180^o\\ \Rightarrow \widehat{C}=60^o\Rightarrow \widehat{B}=2\widehat{C}=2.60^o=120^o[/tex]
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