Quan sát Hình 9. c) Chứng tỏ rằng

c) Ta có $\widehat{aOc}$  và $\widehat{bOd}$  là hai góc đối đỉnh nên $\widehat{aOc}=\widehat{bOd}$

Khi đó $$\begin{gathered} \widehat{aOg}+\widehat{cOe}+\widehat{bOd}=\widehat{gOa}+\widehat{cOe}+\widehat{aOc} \\ =(\widehat{gOa}+\widehat{aOc})+\widehat{cOe}=\widehat{gOc}+\widehat{cOe}=\widehat{gOe}=180° \end{gathered}$$

Vậy $\widehat{aOg}+\widehat{cOe}+\widehat{bOd}=180°.$