Áp dụng tính chất DTSBN:
\[\dfrac{\widehat{A}}{1}= \dfrac{\widehat{B}}{2}= \dfrac{\widehat{C}}{3}= \dfrac{\widehat{A}+\widehat{B}+\widehat{C}}{1+2+3}= \dfrac{180^{\circ}}{6}=30^{\circ}\]
\[\rightarrow \left\{\begin{matrix} \widehat{A}=30^{\circ} & \\ \widehat{B}=30^{\circ}\times 2=60^{\circ} & \\ \widehat{C}=30^{\circ}\times 3= 90^{\circ} & \end{matrix}\right.\]