ข้อสอบคณิต O-NET - 2553

$$\begin{gathered} \mathrm{จากโจทย์} \\ A = \left\{1, 2, 3, 4, 5, 6, 7, 8, \dots \right\} \\ B = \left\{\left\{1, 2\right\}, \left\{3, 4, 5\right\}, 6, 7, 8, \dots \right\} \\ -\mathrm{จะได้} A-B = \left\{1, 2, 3, 4, 5\right\} \\ \mathrm{ดังนั้น} n\left(A-B\right) = 5 \\ \mathrm{ข้อ} 1 \mathrm{ถูก} \end{gathered}$$

$$\begin{gathered} -\mathrm{จะได้} B-A = \left\{\left\{1, 2\right\}, \left\{3, 4, 5\right\}\right\} \\ \mathrm{ดังนั้น} n\left(B-A\right) = 2 \\ \mathrm{แสดงว่า} \mathrm{เพาเวอร์เซต} : 2^{n\left(B-A\right)} \\ = 2^2 = 4 \\ \mathrm{ข้อ} 2 \mathrm{ถูก} \end{gathered}$$

$$\begin{gathered} -\mathrm{จะได้} \left(A-B\right)\cup \left(B-A\right) = \left\{1, 2, 3, 4, 5, \left\{1, 2\right\}, \left\{3, 4, 5\right\}\right\} \\ \mathrm{ดังนั้น} n\left[\left(A-B\right)\cup \left(B-A\right)\right] = 7 \mathrm{เป็นจำนวนคี่} \\ \mathrm{ข้อ} 3 \mathrm{ผิด} \\ -\mathrm{จะได้} A\cap B = \left\{6, 7, 8, \dots \right\} \\ \mathrm{ข้อ} 4 \mathrm{ถูก} \end{gathered}$$