8.
\((1)\) 已知\(a∈R\),\(i\)为虚数单位,若\(\dfrac{a-i}{2+i} \)为实数,则\(a\)的值为______ .
\((2)\)观察下面的数阵,则第\(40\)行最左边的数是______.
\((3)\)已知在极坐标系下,点\(A\left(2, \dfrac{π}{6}\right) \),\(B\left(4, \dfrac{2π}{3}\right) \),\(O\)是极点,则\(\triangle AOB\)的面积等于______.
\((4)\)点\(P\)的柱坐标为\((\sqrt{2} ,\dfrac{π}{4} ,1)\),写出点\(P\)直角坐标______ .