Ta có: \(\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^o\)
\(\Rightarrow\widehat{A}+120^o+60^o+90^o=360^o\)
\(\Rightarrow\widehat{A}=360^o-90^o-60^o-120^o=90^o\)
Trong tứ giác ABCD ta có:
\(\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^o\)
\(\Rightarrow\widehat{A}=360^o-\left(\widehat{B}+\widehat{C}+\widehat{D}\right)=360^o-\left(120^o+60^o+90^o\right)\)
\(\Rightarrow\widehat{A}=90^o\)
Vậy,...