Given $a, b>0$, show that $\sqrt{a b} \leq \frac{1}{2}(a+b)$ (this is the arithmetic-geometric mean inequality). Generalize this to $\left(a_1 \cdot a_2 \cdots a_n\right)^{1 / n} \leq(1 / n)\left(a_1+a_2+\cdots+a_n\right)$. [Hint: Induction and Bernoulli's inequality.]