3^{k + 1} - 1 &= 3 \times 3^k - 1 \\
"Engagement rangers have also been out in the landscape over the past decade, encouraging people to care for the landscape, including keeping dogs on leads, picking up dog poo and sticking to pathways to avoid disturbing the nightjars and other wildlife."。关于这个话题，Telegram 官网提供了深入分析
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