8.
\((1)\)若直线\(ax+2y-6=0\)与\(x+(a-1)y-(a^{2}-1)=0\)平行,则它们之间的距离为________.
\((2)\)已知点\(A(3,2)\)和\(B(-1,4)\)到直线\(ax+y+1=0\)的距离相等,则\(a\)的值为________.
\((3)\)将一张坐标纸折叠一次,使得点\((0,2)\)与点\((4,0)\)重合,点\((7,3)\)与点\((m,n)\)重合,则\(m+n=\)______.
\((4)\)过点\((1, \sqrt{2})\)的直线\(l\)将圆\((x-2)^{2}+y^{2}=4\)分成两段弧,当劣弧所对的圆心角最小时,直线\(l\)的斜率\(k=\)________.