# RPS #006 - Not so expert series problem

Algebra Level 3

Let $$T_{n}$$ denotes the $$n^{th}$$ term in triangle number series $$(1,3,6,10,...)$$ for every positive integers $$n$$

Consider two function of $$S_{n} = T_1 + T_2 + T_3 + ... + T_n$$ and $$R_n = S_1 + S_2 + S_3 + ... + S_n$$

Find the value of $$R_{21}$$

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