1.
\((1)\)因式分解:\(9{{a}^{3}}b-ab\)__________________________________
\((2)\)分式\(\dfrac{1}{\sqrt{2-x}}\)有意义时,\(x\)的取值范围是_____.
\((3)\)如图,在平面直角坐标系中,\(⊙P\)的圆心是\((2,a)(a > 2)\),半径为\(2\),函数\(y=x\)图象被\(⊙P\)所截得的弦\(AB\)的长为\(2\sqrt{3}\),则\(a\)的值是____________.
\((4)\)如图,在\(Rt\triangle ABC\)中,\(∠A=90^{\circ}\),\(AB=AC\),\(BC=20\),\(DE\)是\(\triangle ABC\)的中位线,点\(M\)是边\(BC\)上一点,\(BM=3\),点\(N\)是线段\(MC\)上的一个动点,连接\(DN\),\(ME\),\(DN\)与\(ME\)相交于点\(O.\)若\(\triangle OMN\)是直角三角形,则\(DO\)的长是_______________.