\((1)\)已知\(\triangle ABC\)的三边长分别为\(3\),\(5\),\(7\),则该三角形的外接圆半径等于_________.
\((2)\)在\(\triangle \)\(ABC\)中,\(∠A= \dfrac{2π}{3} \) ,\(a= \sqrt{3}c \),则\( \dfrac{b}{c} =\)_________.
\((3)\)观察数列\( \dfrac{3}{2},- \dfrac{8}{3}, \dfrac{15}{4},- \dfrac{24}{5}, \dfrac{35}{6},- \dfrac{48}{7}… \)的一个通项公式是________
\((4)\)在\(\triangle ABC\)中,角\(A\),\(B\),\(C\)所对的边分别是\(a\),\(b\),\(c\),若\({b}^{2}+{c}^{2}={a}^{2}-bc \),且\( \overset{→}{AC}· \overset{→}{AB}=-4 \),则\(\triangle ABC\)的面积等于 .