ข้อสอบคณิต O-NET - 2563

$$\begin{gathered} \mathrm{จากโจทย์} \\ \mathrm{จะได้} A = \left\{-1, 1, 3, 5, 7, 9\right\} \\ B = x^2-9 < 0 \\ \left(x-3\right)\left(x+3\right) < 0 \end{gathered}$$

$$\begin{gathered} B = \left\{-2, -1, 0, 1, 2\right\} \\ \mathrm{จะได้} A-B = \left\{-1, 1, 3, 5, 7, 9\right\} - \left\{-2, -1, 0, 1, 2\right\} \\ = \left\{3, 5, 7, 9\right\} \\ \mathrm{ดังนั้น} \mathrm{ตอบข้อ} 4 \end{gathered}$$