Ta có:
\(\widehat{C_{ng}}+\widehat{C}=180^o\) (kề bù)
\(\Rightarrow\widehat{C}=180^o-\widehat{C_{ng}}\)
\(\Rightarrow\widehat{C}=180^o-120^o=60^o\)
Mà: \(\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^o\)(tổng các góc của tứ giác)
\(\Rightarrow\widehat{D}=360^o-\left(\widehat{A}+\widehat{B}+\widehat{C}\right)\)
\(\Rightarrow\widehat{D}=360^o-\left(130^o+90^o+60^o\right)=80^o\)
Vậy: ...
\(\widehat{C}=180^o-120^o=60^o\left(kề.bù\right)\)
\(\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^o\)
\(\Rightarrow\widehat{D}=360^o-\left(\widehat{A}+\widehat{B}+\widehat{C}\right)\)
\(\Rightarrow\widehat{D}=360^o-\left(130^0+90^o+60^o\right)\)
\(\Rightarrow\widehat{D}=80^o\)
