9.
已知底面为边长为\(2\)的正方形,侧棱长为\(1\)的直四棱柱\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,\(P\)是面\(A_{1}B_{1}C_{1}D_{1}\)上的动点\(.\)给出以下四个结论中,正确的个数是\((\) \()\)
\(①\)与点\(D\)距离为\( \sqrt {3}\)的点\(P\)形成一条曲线,则该曲线的长度是\( \dfrac {π}{2}\);
\(②\)若\(DP/\!/\)面\(ACB_{1}\),则\(DP\)与面\(ACC_{1}A_{1}\)所成角的正切值取值范围是\([ \dfrac { \sqrt {6}}{3},+∞)\);
\(③\)若\(DP= \sqrt {3}\),则\(DP\)在该四棱柱六个面上的正投影长度之和的最大值为\(6 \sqrt {2}\).