Sea $\{ a_1, a_2, \ldots, a_n\} = \{1, 2, \ldots, n\}$ . Demuestra que \[\frac 12 +\frac 23 +\cdots+\frac{n-1}{n} \leq \frac{a_1}{a_2} + \frac{a_2}{a_3} +\cdots+\frac{a_{n-1}}{a_n}.\]