By Long Luo

$f(x) = \sum _{n=0}^{\infty}{\frac{f^{(n)}(a)}{n!}}(x-a)^{n} = f(a) + {\frac {f'(a)}{1!}}(x - a) + {\frac {f''(a)}{2!}}(x - a)^{2} + {\frac {f'''(a)}{3!}}(x - a)^{3} + \cdots$

# 几何级数 Geometric series

$\frac{1}{1 - x} = \sum _{n=0}^{\infty}x^{n} = 1 + x + x^{2} + \cdots + x^{n}$

$$-x$$ 代入 $$x$$ 上式，则：

$\frac{1}{1 + x} = \sum _{n=0}^{\infty}(-1)^nx^{n} = 1 - x + x^{2} - x^3 + \cdots + (-1)^n x^{n}$

$$x^2$$ 替代 $$x$$ , 由于 $$\arctan x = \int_{0}^{x} \frac{1}{1 + x^2} \mathrm{d}x$$ ，对于 $$-1 \le x \le 1, x \neq \pm i$$

$\arctan x = \sum _{n=0}^{\infty }{\frac {(-1)^{n}}{2n + 1}}x^{2n + 1} = x - {\frac {x^3}{3}} + {\frac {x^5}{5}} - \cdots + \frac{(-1)^n}{2n + 1}x^{2n + 1}$

\begin{aligned} \frac {1}{(1-x)^2} &= \sum _{n=1}^{\infty }n x^{n-1} \\ &= 1 + 2x + 3x^2 + \cdots + n x^{n-1} \end{aligned}

$$\frac{1}{(1 - x)^3} = \frac{1}{2} (\frac{1}{(1 - x)^2})'$$ ，则有：

$\frac {1}{(1 - x)^3} = \sum _{n=2}^{\infty }{\frac {n(n - 1)}{2}}x^{n - 2}$

# 指数函数 Exponent function

$e^x = \sum _{n=0}^{\infty }{\frac{x^n}{n!}} = 1 + x + {\frac{x^2}{2!}} + {\frac {x^3}{3!}} + \cdots + {\frac{x^n}{n!}}$

\begin{aligned} (e^x)' &= (\frac{1}{0!}+\frac{x}{1!}+\frac{x^2}{2!}+\frac{x^3}{3!}+\cdots)' \\ e^x &= 0+1+\frac{x}{1}+\frac{x^2}{2!}+\frac{x^3}{3!}\cdots \\ &= 1+x+\frac{x^2}{2!}+\frac{x^3}{3!}+\cdots \end{aligned}

\begin{aligned} a^x &= e^{x \ln a} \\ &= 1 + x \ln a + \frac{(x \ln a)^2}{2!} + \frac{(x \ln a)^3}{3!} + \cdots + \frac{(x \ln a)^n}{n!} \\ \end{aligned}

# 三角级数 Trigonometric functions

$$\sin x$$奇函数，只有奇数项$$\sin 0 = 1$$ ，同时 $$-1 \le \sin x \le 1$$ ，所以不同多次项的正负号要依次出现：

$\sin x = \sum _{n=0}^{\infty} \frac{(-1)^{n}}{(2n + 1)!} x^{2n + 1} = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \cdots + (-1)^n\frac{x^{2n + 1}}{(2n + 1)!}$

$$\sin x$$ 求导为 $$\cos x$$ ，同时 $$\cos x$$偶函数，只有偶数项$$\cos 0 = 1$$

\begin{aligned} \cos x &= \frac{\mathrm{d} \sin x}{\mathrm{d} x} \\ &= 1 - \frac{x^2}{2!} + \frac{x^4}{4!} - \frac{x^6}{6!} + \cdots + (-1)^n\frac{x^{2n}}{(2n)!} \\ \end{aligned}

# 自然对数 Natural logarithm

$\frac{1}{1 - x} = \sum _{n=0}^{\infty}x^{n} = 1 + x + x^{2} + \cdots + x^{n} \quad \forall x \in (-1, 1)$

$\frac{1}{x} = \sum _{n=0}^{\infty}(1 - x)^{n} = 1 - (x - 1) + (x - 1)^{2} + \cdots + (-1)^n(x - 1)^{n} \quad \forall x \in (0, 2)$

$\frac{-1}{1-x} = -(1 - x)^{-1} = -(1 + x + x^2 + x^3 + \cdots + x^n)$

$\ln (1 - x) = -x - \frac {x^2}{2} - \frac {x^3}{3} - \frac {x^4}{4} - \cdots$

$\ln (x) = \sum_{n = 1}^{\infty} (-1)^{n - 1} \frac{(x - 1)^n}{n} = (x - 1) - {\tfrac {1}{2}}(x - 1)^{2} + {\tfrac {1}{3}}(x - 1)^{3} - {\tfrac {1}{4}}(x - 1)^{4} + \cdots$

\begin{aligned} \ln(1 - x) &= -\sum _{n=1}^{\infty}{\frac{x^n}{n}} \\ &= -x - \frac {x^2}{2} - \frac {x^3}{3} -\cdots - \frac {x^n}{n} \end{aligned}

$$0 < x \le 1$$$$\ln x = \ln (1 + (x - 1))$$ ，则：

$\ln x = \sum _{n=1}^{\infty}{\frac{(-1)^{n + 1}}{n}}x^{n} = (x - 1) - \frac{(x - 1)^2}{2} + \frac {(x - 1)^3}{3} - \cdots + \frac {(-1)^{n + 1}}{n}x^n$

# 二项式 Binomial series

$$-1 \le x \le 1$$ 时，对于任意 $$\alpha \in \mathbb {C}$$ ，由二项式定理6 即可得:

\begin{aligned} (1 + x)^{\alpha} &= \sum _{n=0}^{\infty}{\binom{\alpha}{n}}x^{n} \\ &= 1 + \alpha x + \frac{\alpha(\alpha - 1)}{2!}x^2 + \cdots + \frac{\alpha (\alpha - 1) \cdots (\alpha - n + 1)}{n!}x^{n} \end{aligned}

${\binom {\alpha}{n}} = \prod _{k = 1}^{n}{\frac{\alpha - k + 1}{k}} = {\frac {\alpha (\alpha - 1) \cdots (\alpha - n + 1)}{n!}}$