$\textit{Ta có: $\widehat{A'OB'}$ = $\widehat{AOB}$ = $72^\circ$ (đối đỉnh)}$
$\textit{Ta có: $\widehat{A'OB'}$ + $\widehat{AOB'}$ = $180^\circ$ (kề bù)}$
$\textit{Thay số: $\widehat{AOB'}$ = $180^\circ$ - $72^\circ$}$
$\textit{=> $\widehat{AOB'}$ = $108^\circ$}$