群数列 220425

問題
数列 {a_n} を
\(\frac{1}{2}\), \(\frac{1}{3}\), \(\frac{2}{3}\), \(\frac{1}{4}\), \(\frac{2}{4}\), \(\frac{3}{4}\), \(\frac{1}{5}\), \(\frac{2}{5}\), \(\frac{3}{5}\), \(\frac{4}{5}\), \(\frac{1}{6}\), \(\frac{2}{6}\), ……
と定めるとき、a₂₂₀ を求めよ。