\(\widehat{DAB}+\widehat{ADC}=180^0\)
mà \(\widehat{DAB}=3\widehat{ADC}\)\(\Rightarrow\widehat{ADC}=45^0\Rightarrow\widehat{DAB}=135^0\)
\(\widehat{ABC}+\widehat{BCD}=180^0\) mà lại có: \(\widehat{ABC}-\widehat{BCD}=30^0\)
\(\Rightarrow\widehat{BCD}=75^0\Rightarrow\widehat{ABC}=105^0\)