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321.
n-th terem of an arithmetic sequence
$a_n = a_1 + (n-1)\cdot d$
322.
Sum of the first n terms
$S_n=\dfrac{n}{2}\left[2a_1 + (n-1)\cdot d\right]$
323.
Sum of first n terms
$S_n= \dfrac{n}{2}\left(a_1+a_n\right)$
324.
n-th term
$a_n=\dfrac{a_{n-1}+a_{n+1}}{2}$
325.
n-th term
$a_n=a_1\cdot q^{n-1}$
326.
Sum of first n terms
$S_n = a_1\frac{q^n-1}{q-1}$
327.
Sum of first n terms
$S_n = \dfrac{a_nq-a_1}{q-1}$
328.
Common raito
$q=\dfrac{a_n}{a_1}$
329.
$\dfrac{1}{1-x} = 1 + x + x^2 + x^3 + \cdots \quad -1 < x < 1$
330.
$\dfrac{1}{(1+x)^2} = 1 - 2x + 3x^2 - 4x^3 + \cdots \quad -1 < x < 1$
331.
$\dfrac{1}{(1-x)^2} = 1 + 2x + 3x^2 + 4x^3 + \cdots \quad -1 < x < 1$
332.
$\dfrac{1}{(1+x)^3} = 1 - 3x + 6x^2 - 10x^3 + \cdots \quad -1 < x < 1$
333.
$e^x = 1 + x + \dfrac{x^2}{2!} + \dfrac{x^3}{3!} + \cdots$
334.
$a^x = 1 + x\,\ln a + \dfrac{(x\,\ln a)^2}{2!} + \dfrac{(x\,\ln a)^3}{3!} + \cdots$
335.
$\ln(1+x) = x - \dfrac{x^2}{2} + \dfrac{x^3}{3} - \dfrac{x^4}{4} + \cdots \quad -1 < x \leq 1$
336.
$\sin x = x - \dfrac{x^3}{3!} + \dfrac{x^5}{5!} - \dfrac{x^7}{7!} + \cdots$
337.
$\cos x = 1 - \dfrac{x^2}{2!} + \dfrac{x^4}{4!} - \dfrac{x^6}{6!} + \cdots$
338.
$\arcsin x = x + \dfrac{1}{2}\dfrac{x^3}{3} + \dfrac{1 \cdot 3}{2 \cdot 4} \frac{x^5}{5} + \dfrac{1\cdot3\cdot5}{2\cdot4\cdot6}\frac{x^7}{7} + \cdots \quad -1 < x <1$
339.
$\coth x = \dfrac{1}{x} + \dfrac{x}{3} - \dfrac{x^3}{45} + \cdots \dfrac{(-1)^{n-1} 2^{2n}B_nx^{2n-1}}{(2n)!} + \cdots \quad 0 < |x| < \pi$
340.
$\tanh x = x - \dfrac{x^3}{3} + \dfrac{2x^5}{15} + \cdots \dfrac{(-1)^{n-1} 2^{2n}\left(2^{2n}-1\right)B_nx^{2n-1}}{(2n)!} + \cdots \quad |x| < \frac{\pi}{2}$