Gọi giao điểm như hình bên.
Ta thấy: [tex]\left\{\begin{matrix} \widehat{MKL}=\widehat{D}+\widehat{DJK}=\widehat{D}+\widehat{E}+\widehat{F}\\ \widehat{AMK}=\widehat{B}+\widehat{C}\\ \widehat{KLI}=\widehat{G}+\widehat{H} \end{matrix}\right.\Rightarrow \widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}+\widehat{E}+\widehat{F}+\widehat{G}+\widehat{H}+\widehat{I}=\widehat{A}+\widehat{MKL}+\widehat{AMK}+\widehat{KLI}+\widehat{I}=540^o[/tex]