3.
\((1)\)已知随机变量\(X\)等可能地取\(1\),\(2\),\(3\),\(…\),\(n\),若\(P(X < 5)=0.2\),则\(n\)的值为____.
\((2)\)在正方体\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,直线\(BC_{1}\)与平面\(A_{1}BD\)所成的角的正弦值是______.
\((3)\)知\(F\)是双曲线 \( \dfrac{x^{2}}{4}- \dfrac{y^{2}}{12}=1\)的左焦点,\(A(1,\sqrt{7})\),\(P\)是双曲线右支上的动点,则\(|PF|+|PA|\)的最小值为___________
\((4)\)已知函数\(f(x)=\begin{cases} x^{2}+2x+a,x < 0, \\ \ln x,x > 0, \end{cases}\)其中\(a\)是实数,设\(A(x_{1},f(x_{1}))\),\(B(x_{2},f(x_{2}))\)为该函数图象上的两点,且\(x_{1} < x_{2}.\)若函数\(f(x)\)的图象在点\(A\),\(B\)处的切线重合,则实数\(a\)的取值范围是______.