ข้อสอบคณิต O-NET - 2551

$$\begin{gathered} \mathrm{จากโจทย์} \\ {S}_{10} = a_1+a_2+a_3+\dots +a_{10} \\ S_8 = a_1+a_2+a_3+\dots +a_8 \end{gathered}$$

$$\begin{gathered} \mathrm{จะได้} S_{10}-S_8 = a_9+a_{10} =32 \\ = a_9+a_{9}r = 32 \\ = a_9+2a_9 = 32 \\ 3a_9 = 32 \\ \mathrm{ดังนั้น} a_9 = \frac{32}{3} \end{gathered}$$