Ta có :
\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^{\circ}
<=> \widehat{A}+\widehat{B}=360^{\circ}-(\widehat{D}+\widehat{C})
<=>\widehat{A}+\widehat{B}=360^{\circ}-2.(180^{\circ}-\widehat{ODC}-\widehat{OCD})
<=>\widehat{A}+\widehat{B}=2.\widehat{COD}...