ข้อสอบคณิต O-NET - 2563

$$\begin{gathered} \mathrm{จากโจทย์} \\ f\left(x\right) = {\left(x+a\right)}^2-a \\ \mathrm{จะได้} f\left(-2\right) = {\left(-2+a\right)}^2-a \\ f\left(4\right) = {\left(4+a\right)}^2-a \\ {\left(-2+a\right)}^2\cancel{-a} = {\left(4+a\right)}^2\cancel{-a } \\ 4-4a+\cancel{a^2} = 16+8a+\cancel{a^2} \\ -12 = 12a \\ a = -1 \to \mathrm{ตอบข้อ} 3 \end{gathered}$$