[tex]\it a)\ AB=AC \Rightarrow \Delta ABC-isoscel\ \ \ \ \ (*)\\ \\ \\ b)\ \ m(\widehat{A})=\dfrac{m(\stackrel\frown{BC})}{2}=\dfrac{100^o}{2}=50^o\ \stackrel{(*)}{\Longrightarrow} m(\widehat{C})=m(\widehat{A})=50^o\\ \\ \\ m(\widehat{B})=180^o-2\cdot50^o=80^o[/tex]