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507.
Sum of first n odd squares
$1^2 + 3^2 + 5^2 + \ldots + (2n-1)^2 = \dfrac{n(4n^2-1)}{3}$
509.
$\tan x = x + \dfrac{x^3}{3} + \dfrac{2x^5}{15} + \cdots + \dfrac{2^{2n}\left(2^{2n}-1\right)B_nx^{2n-1}}{(2n)!} \quad -\frac{\pi}{2} < x < \frac{\pi}{2}$