[tex]\displaystyle\bf\\Folosim~teorema~sinusurilor.\\BC=a~~deoarece~este~latura~opusa~unghiului~A\\\\\frac{a}{sinA}=2R~~~unde~R=raza~cercului~circumscris.\\\\Rezolvare:\\\\\frac{a}{sinA}=2R\\\\\frac{a}{sin\,30^o}=2R\\\\\frac{4}{\dfrac{1}{2} }=2R\\\\4\times2=2R\\\\R=\frac{4\times2}{2}\\\\\boxed{\bf~R=4~cm}[/tex]
