Do AB // CD ( GT )
\(\Rightarrow\widehat{A}+\widehat{C}=180^o\)
\(\Rightarrow2\widehat{C}+\widehat{C}=180^o\)
\(\Rightarrow3\widehat{C}=180^o\)
\(\Rightarrow\widehat{C}=60^o\)
\(\Rightarrow\widehat{A}=60^o.2=120^o\)
Do ABCD là hình thang cân
\(\Rightarrow\widehat{C}=\widehat{D}\)
Mà \(\widehat{C}=60^o\)
\(\Rightarrow\widehat{D}=60^o\)
AB // CD \(\Rightarrow\widehat{D}+\widehat{B}=180^o\)
\(\Rightarrow\widehat{B}=180^o-60^o=120^o\)
Vậy \(\widehat{A}=\widehat{B}=120^o;\widehat{C}=\widehat{D}=60^o\)