ข้อสอบคณิต O-NET - 2553

$$\begin{gathered} \mathrm{จากโจทย์} a_n = 3+{\left(-1\right)}^n \\ \mathrm{จะได้} a_1 = 3+{\left(-1\right)}^1 = 2 \\ a_2 = 3+{\left(-1\right)}^2 = 4 \\ a_3 = 3+{\left(-1\right)}^3 = 2 \\ a_4 = 3+{\left(-1\right)}^4 = 4 \\ ⋮ \\ a_{40} = 3+{\left(-1\right)}^{40} = 4 \end{gathered}$$

$$\begin{gathered} \mathrm{ดังนั้น} \mathrm{จะมีอีกครึ่งหนึ่งของ} 40 \mathrm{พจน์แรก} \\ \mathrm{ตอบ} \mathrm{ข้อ} 2 \end{gathered}$$