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            • 1.,则= ______
            • 2.
              已知\(A\),\(B\),\(C\)三点都在体积为\( \dfrac {500π}{3}\)的球\(O\)的表面上,若\(AB=4\),\(∠ACB=30^{\circ}\),则球心\(O\)到平面\(ABC\)的距离为 ______ .
            • 3. 如图所示,在空间直角坐标系中\(BC=2\),原点\(O\)是\(BC\)的中点,点\(A\)的坐标是\(( \dfrac { \sqrt {3}}{2}, \dfrac {1}{2},0)\),点\(D\)在平面\(yOz\)上,且\(∠BDC=90^{\circ}\),\(∠DCB=30^{\circ}\),则向量\( \overrightarrow{AD}\)的坐标为\((\)  \()\)
              A.\((- \dfrac { \sqrt {3}}{2},- \dfrac {1}{2}, \dfrac { \sqrt {3}}{2})\)
              B.\((- \dfrac { \sqrt {3}}{2},-1, \dfrac { \sqrt {3}}{2})\)
              C.\((- \dfrac {1}{2},- \dfrac { \sqrt {3}}{2}, \dfrac { \sqrt {3}}{2})\)
              D.\(( \dfrac { \sqrt {3}}{2},1, \dfrac { \sqrt {3}}{2})\)
            • 4.

              空间四边形\(ANCD\)中,若向量\( \overrightarrow{AB}=(-3,5,2) \),\( \overrightarrow{CD}=(-7,-1,-4) \),点\(E\),\(F\)分别为线段\(BC\),\(AD\)的中点,则\( \overrightarrow{EF} \)的坐标为(    ) 

                

              A.\((2,3,3)\)       
              B.\((-2,-3,-3)\)
              C.\((5,-2,1)\)     
              D.\((-5,2,-1)\)
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