ข้อสอบคณิต O-NET - 2557

$$\begin{gathered} \mathrm{จากสูตรลำดับเรขาคณิต} \\ a_n = a_1{r}^{n-1} , S_n = \frac{a_{n}r-a_1}{r-1} \\ \mathrm{จะได้} a_{10} = a_1{r}^9 \\ = \frac{1}{2}{r}^9 = 256 \\ r = 2 \end{gathered}$$

$$\begin{gathered} \mathrm{จะได้} S_{10} = \frac{a_{10}r-a_1}{r-1} \\ = \frac{256\left(2\right)-{\displaystyle \frac{1}{2}}}{2-1} \\ = 511.5 \\ \mathrm{ดังนั้น} \mathrm{ตอบข้อ} 2 \end{gathered}$$