a) Ta có: $\widehat{A}+\widehat{B}+\widehat{C}=180^{\circ} =>\widehat{B}+\widehat{C}=180^{\circ}-\widehat{A}=180^{\circ}-60^{\circ}=120^{\circ}$.
Vì $\widehat{B}>\widehat{A}=60^{\circ}$ nên $\widehat{C}<60^{\circ}$. Như vậy $\widehat{C}<\widehat{A}<\widehat{B}$.
b) Ta có: $\widehat{A}+\widehat{B}+\widehat{C}=180^{\circ} =>\widehat{B}+\widehat{C}=180^{\circ}-\widehat{A}=180^{\circ}-55^{\circ}=125^{\circ}$.
Vì $\widehat{B}<\widehat{A}=55^{\circ}$ nên $\widehat{C}>70^{\circ}$. Như vậy $\widehat{B}<\widehat{A}<\widehat{C}$.