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501.
Sum of the first n numbers
$1+2+3+ \cdots + n = \dfrac{n(n+1)}{2}$
502.
Sum of the first n even numbers
$2+4+6+ \cdots + 2n = n(n+1)$
503.
Sum of the first n odd numbers
$1+3+5+\ldots+(2n-1) = n^2$
504.
$k + (k+1) + \ldots + (k+n-1) = \dfrac{n(2k+n-1)}{2}$
505.
Sum of first n squares
$1^2+2^2+3^2+ \ldots + n^2 = \dfrac{n(n+1)(2n+1)}{6}$
506.
Sum of first n cubes
$1^3+2^3+3^3+ \ldots + n^3 = \left(\dfrac{n(n+1)}{2}\right)^2$
507.
Sum of first n odd squares
$1^2 + 3^2 + 5^2 + \ldots + (2n-1)^2 = \dfrac{n(4n^2-1)}{3}$
508.
Sum of first n odd cubes
$1^3+3^3+5^3+ \ldots + (2n-1)^3 = n^2(2n^2-1)$
509.
$\tan x = x + \dfrac{x^3}{3} + \dfrac{2x^5}{15} + \cdots + \dfrac{2^{2n}\left(2^{2n}-1\right)B_nx^{2n-1}}{(2n)!} \quad -\frac{\pi}{2} < x < \frac{\pi}{2}$
510.
$\dfrac{1}{1+x} = 1 - x + x^2 -x^3 + \cdots \quad -1 < x < 1$
511.
Circle perimeter
$P=2r\pi $
512.
Circle area
$A = r^2\pi$
513.
Circle area - version 2
$A = \dfrac{\pi d^2}{4}$
514.
Circle area -verison 3
$ A = \dfrac{P \cdot r }{2}$
515.
Circle arc - verison 1
$ s = r \cdot x $
516.
Circle arc
$ s = \dfrac{\pi r \alpha}{180\degree} $
517.
Sector area
$ B = \dfrac{r\cdot s}{2} $
518.
Sector area - version 2
$ B = \dfrac {r^2 \cdot x}{2} $
519.
Sector area - version 2
$ B = \dfrac{\pi \cdot r^2 \cdot \alpha}{360\degree} $
520.
Circle chord
$ a = 2\sqrt{2hr-h^2} $