8.
\((1)\) 直线\(x+2ay-1=0\)与直线\((a-1)x-ay-1=0\)平行,则\(a\)的值是_________.
\((2)\) 在面积为\(S\)的\(\triangle ABC\)的边\(AB\)上任取一点\(P\),则\(\triangle PBC\)的面积不小于\( \dfrac{S}{3}\)的概率是_________
\((3)\)已知直线\(l\):\(x- \sqrt{3}y+6=0 \)与圆\(x^{2}+y^{2}=12\)交于\(A\),\(B\)两点,过\(A\),\(B\)分别作\(l\)的垂线与\(x\)轴交于\(C\),\(D\)两点,则\(\left|CD\right|= \)_____________.
\((4)\)在平面直角坐标系\(xoy\)中,直线\(y=-x+2\)与圆\({{x}^{2}}+{{y}^{2}}={{r}^{2}}(r > 0)\)交于\(A\),\(B\)两点,\(O\)为坐标原点,若圆上有一个\(C\)满\(\overset{\to }{{OC}}\,=\dfrac{5}{4}\overset{\to }{{OA}}\,+\dfrac{3}{4}\overset{\to }{{OB}}\,\),则\(r=\)______________.