如何证明算数平均值不大于二次平均值?

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$$\begin{align}\sqrt{\frac{a_1^2 + \cdots + a_k^2}{k}} & \ge \frac{ a_1 + a_2 + \cdots + a_n }{n} \\f_{\text{QM}} & \ge f_{\text{AM}} \end{align}$$

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