7.
\((1)\)已知平面向量\( \overset{→}{a}=(2,-1), \overset{→}{b}=(m,2) \),且\( \overset{→}{a}⊥ \overset{→}{b} \),则\(| \overset{→}{a}+2 \overset{→}{b}|= \)__________.
\((2)\)设\(0\leqslant x < 2\pi \),且\(\sqrt{1-{\sin }2x}={\sin }x-{\cos }x\),则\(x\)的取值范围是________.
\((3)\)直线\(l:x{\sin }\alpha +y-1=0\left( \alpha \in R \right)\),则直线\(l\)的倾斜角的取值范围为___________
\((4)\)如图,在正方体\(ABCD-{{A}_{1}}{{B}_{1}}{{C}_{1}}{{D}_{1}}\)中,棱长为\(1\),点\(P\)为线段\({{A}_{1}}C\)上的动点\((\)包含线段端点\()\),则下列结论正确的______.
\(①\)当\( \overset{→}{{A}_{1}C}=3 \overset{→}{{A}_{1}P} \)时,\({{D}_{1}}P/\!/\)平面\(BD{{C}_{1}}\);
\(②\)当\( \overset{→}{{A}_{1}C}=3 \overset{→}{{A}_{1}P} \)时,\({{A}_{1}}C\bot \)平面\({{D}_{1}}AP\);
\(③\angle AP{{D}_{1}}\)的最大值为\({{90}^{\circ }}\);
\(④AP+P{{D}_{1}}\)的最小值为\(\dfrac{2\sqrt{6}}{3}\).