# Inverse trig functions!

Geometry Level 3

$\cot^{-1}(3) + \cot^{-1} (7) + \cot^{-1} (13) + \cot^{-1}(21) + \ldots = \ ?$

Clarification: $3,7,13,21,\ldots$ is an increasing progression of integers for which the difference between the $n$-th and $(n-1)$-th term is equals to $2n$.

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