Gaokao Math Practice Question
5. (5 分) 等差数列 $\left\{a_{n}\right\}$ 的公差为 2 , 若 $a_{2}, a_{4}, a_{8}$ 成等比数列, 则 $\left\{a_{n}\right\}$ 的前 $n$ 项和 $S_{n}=(\quad)$
- A.$n(n+1)$
- B.$n(n-1)$
- C.$\frac{\mathrm{n}(\mathrm{n}+1)}{2}$
- D.$\frac{n(n-1)}{2}$
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