№1980
0
\[\begin{array}{l} {\text{Пусть }}{a_k} = \frac{{\left( {2k + 1} \right)\left( {2k + 2} \right)}}{{{k^2}}}. \hfill \\ \frac{{{\pi ^2}}}{9} = 1 + \frac{{1 + \frac{{1 + \frac{{1 + ...}}{{{a_3}}}}}{{{a_2}}}}}{{{a_1}}}. \hfill \\ \end{array}\]
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