ข้อสอบคณิต O-NET - 2557

$$\begin{gathered} \mathrm{จากโจทย์} \mathrm{จะได้} \\ = \sqrt{a+\frac{1}{a}-2} = \sqrt{\frac{\sqrt{5}+2}{\sqrt{5}-2}+\frac{\sqrt{5}-2}{\sqrt{5}+2}-2} \\ = \sqrt{\frac{\left(5+4\sqrt{5}+4\right)+\left(5-4\sqrt{5}+4\right)}{5-4}-2} \\ = \sqrt{18-2} \\ = \sqrt{16} \\ = 4 \end{gathered}$$