1/1 2 3 1/1 2 3 4 ... 1/1 2 .. 59

Lời giải:

\(\frac{1}{{1 + 2 + 3 + ... + n}} = \frac{1}{{n(n + 1):2}} = \frac{2}{{n(n + 1)}} = \frac{2}{n} - \frac{2}{{n + 1}}\)

Vế trái = \(\frac{2}{3} - \frac{2}{4} + \frac{2}{4} - \frac{2}{5} + ... + \frac{2}{{59}} - \frac{2}{{60}} = \frac{2}{3} - \frac{2}{{60}} < \frac{2}{3}\)