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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