8.
\((1)\)在\(\triangle ABC\)中,角\(A\),\(B\),\(C\)所对的边分别为\(a\),\(b\),\(c.\)若\(A=\dfrac{\pi }{3}\),\(\cos B=\dfrac{2\sqrt{7}}{7}\),\(b=2\),则\(a=\)________.
\((2)\)如图所示,已知矩形\(ABCD\)中,\(AB=3\),\(BC=a\),若\(PA⊥\)平面\(ABCD\),在\(BC\)边上取点\(E\),使\(PE⊥DE\),则满足条件的\(E\)点有两个时,\(a\)的取值范围是________.
\((3)\)过点\((6,8)\)的直线与坐标轴正半轴围成三角形面积的最小值为________.
\((4)\)若实数\(a\),\(b\),\(c\)成等比数列,且\(a+b+c=1\),则\(a+c\)的取值范围是________.