ข้อสอบคณิต O-NET - 2563

$$\begin{gathered} \mathrm{จากโจทย์} \\ \frac{a}{b} = -\frac{2}{3} \\ \mathrm{จะได้} \left|\frac{a}{b}\right| = \frac{\left|a\right|}{\left|b\right|} = \frac{2}{3} \end{gathered}$$

$$\begin{gathered} \mathrm{จาก} \left|a\right|+\left|b\right| = 20 \\ \mathrm{หาร} \left|b\right| \mathrm{ทั้งสมการ} \\ \frac{\left|a\right|}{\left|b\right|}+1 = \frac{20}{\left|b\right|} \\ \mathrm{จะได้} \frac{2}{3}+1 = \frac{20}{\left|b\right|} \\ \left|b\right| = 12 \end{gathered}$$

$$\begin{gathered} \mathrm{แทน} b=12 \to a = -8 \\ \mathrm{แทน} b=-12 \to a = 8 \\ \mathrm{ดังนั้น} \left|a+b\right| = 4 \to \mathrm{ตอบข้อ} 1 \end{gathered}$$