دنبالهٔ ${a_n} = \left\{ {\begin{array}{*{20}{c}} {{2^k}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,;\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,n = 3k} \\ { - 2k + 4\,\,\,\,\,\,\,\,\,\,\,\,;\,\,\,\,\,\,\,\,\,\,n = 3k + 1} \\ {\left[ {\frac{n}{{k + 2}}} \right] + a\,\,\,\,\,;\,\,\,\,\,\,\,\,\,\,n = 3k + 2} \end{array}} \right.\,$ به ازای اعداد حسابی $n$، مفروض است. اگر مجموع 10 جملهٔ اول این دنباله 19 باشد، حاصل عبارت $a_2 + a_5 + a_8 + ... + a_{29}$، کدام است؟
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