ข้อสอบคณิต O-NET - 2558

$$\begin{gathered} \mathrm{จากโจทย์} \\ \mathrm{แทน} n = 1, 2, 3, 4, 5 \mathrm{ในสูตร} \\ a_n = \frac{2^n-1}{3n-2} \\ \mathrm{แล้วดูว่าข้อไหนไม่ตรง} \end{gathered}$$

$$\begin{gathered} 1. a_1 = \frac{2^1-1}{3\left(1\right)-2} = \frac{1}{1} = 1 \to \mathrm{ถูก} \\ 2. a_2 = \frac{2^2-1}{3\left(2\right)-2} = \frac{3}{4} \to \mathrm{ถูก} \\ 3. a_3 = \frac{2^3-1}{3\left(3\right)-2} = \frac{7}{7} = 1 \to \mathrm{ถูก} \end{gathered}$$

$$\begin{gathered} 4. a_4 = \frac{2^4-1}{3\left(4\right)-2} = \frac{15}{10} = \frac{3}{2} \to \mathrm{ผิด} \\ 5. a_5 = \frac{2^5-1}{3\left(5\right)-2} = \frac{31}{13} \to \mathrm{ถูก} \\ \mathrm{ดังน้ัน} \mathrm{ตอบข้อ} 4 \end{gathered}$$