a) $\widehat{A}=\widehat{B}=\widehat{C}=180^{\circ}:3=60^{\circ}$;
b) Ta có $\widehat{B}+\widehat{C}=180^{\circ}-\widehat{A}=110^{\circ}$, mà $\widehat{C}-\widehat{B}=20^{\circ}$ nên $\widehat{B}=(110^{\circ}-20^{\circ}):2=45^{\circ}, \widehat{C}=110^{\circ}-45^{\circ}=65^{\circ}$
c)Áp dụng tính chất của dãy tỉ số bằng nhau ta có:
$\frac{\widehat{A}}{1}=\frac{\widehat{B}}{2}=\frac{\widehat{C}}{3}=\frac{\widehat{A}+\widehat{B}+\widehat{C}}{1+2+3}=\frac{180^{\circ}}{6}=30^{\circ}$
Do đó $\widehat{A}=30^{\circ}, \widehat{B}=60^{\circ},\widehat{C}=90^{\circ}$