8.
写出下列数列的一个通项公式
\(① \dfrac{3}{4}, \dfrac{15}{16}, \dfrac{35}{36}, \dfrac{63}{64},··· \) \(②1\dfrac{1}{2},2\dfrac{2}{3},3\dfrac{3}{4},4\dfrac{4}{5},5\dfrac{5}{6},\cdots \)
\(③-1,\sqrt{5},-3,\sqrt{13},-\sqrt{17},\cdots \) \(④0.9,0.99,0.999,0.9999,\cdots \)