Do AB song song DC \(\Rightarrow\widehat{A_1}=\widehat{ADC}\) (hai góc đồng vị)
Mà \(\widehat{ADC}=70^0\Rightarrow\widehat{A_1}=70^0\)
Do \(AB||DC\Rightarrow\widehat{C}+\widehat{B}=180^0\) (hai góc trong cùng phía)
\(\Rightarrow\widehat{C}+115^0=180^0\)
\(\Rightarrow\widehat{C}=180^0-115^0\)
\(\Rightarrow\widehat{C}=65^0\)