ข้อสอบคณิต O-NET - 2563

$$\begin{gathered} \mathrm{จากโจทย์} \\ y = \frac{x-1}{2} \\ x = 2y+1 ; x \mathrm{เป็นจำนวนคู่ไม่ได้} \end{gathered}$$

$$\begin{gathered} \mathrm{จะได้} x = 5 ; y = \frac{5-1}{2} = 2 \notin A \to \mathrm{ผิด} \\ x = 7 ; y = \frac{7-1}{2} = 3 \notin A \to \mathrm{ผิด} \\ x = 9 ; y = \frac{9-1}{2} = 4 \notin A \to \mathrm{ผิด} \end{gathered}$$

$$\begin{gathered} x = 11 ; y = \frac{11-1}{2} = 5 \to \mathrm{ถูก} \\ x = 13 ; y = \frac{13-1}{2} = 6 \to \mathrm{ถูก} \end{gathered}$$

$$\begin{gathered} r = \left\{\left(11, 5\right), \left(13, 6\right)\right\} \\ \mathrm{ดังนั้น} n\left(r\right) = 2 \end{gathered}$$