3:
\(cosA=\dfrac{AB^2+AC^2-CB^2}{2\cdot AB\cdot AC}\)
=>\(15^2+7^2-BC^2=2\cdot15\cdot7\cdot cos30\)
=>\(BC\simeq10\left(cm\right)\)
\(S_{ABCC}=\dfrac{1}{2}\cdot15\cdot7\cdot sin30=\dfrac{1}{4}\cdot15\cdot7=26,25\left(cm^2\right)\)
BC/sinA=2*R
=>2*R=10/sin30=20
=>R=10(cm)