10.
\((1)\)已知复数\(z \)满足\(\left|z\right|- \overset{¯}{z}=2-4i \),则\(z= \)_______.
\((2)\)极坐标系下,直线\(ρ\cos \left(θ- \dfrac{π}{4}\right)= \sqrt{2} \)与圆\(ρ= \sqrt{2} \)的公共点个数是________;
\((3)\)在双曲线\( \dfrac{{x}^{2}}{{a}^{2}}- \dfrac{{y}^{2}}{{b}^{2}}=1\left(a,b > 0\right) \)中,若过双曲线左顶点\(A \)斜率为\(1\)的直线交右支于点\(B \),点\(B \)在\(x \)轴上的射影恰为双曲线的右焦点\(F \),则该双曲线的离心率为 .
\((4)\)若函数\(f(x)=\ln x- \dfrac{1}{2} ax^{2}-2x(a\neq 0)\)存在单调递减区间,则实数\(a\)的取值范围是______.