$$$5 x^{6} - x^{3} + 2 x^{2} + 21$$$ 的積分

求$$$\int \left(5 x^{6} - x^{3} + 2 x^{2} + 21\right)\, dx$$$。

逐項積分：

$${\color{red}{\int{\left(5 x^{6} - x^{3} + 2 x^{2} + 21\right)d x}}} = {\color{red}{\left(\int{21 d x} + \int{2 x^{2} d x} - \int{x^{3} d x} + \int{5 x^{6} d x}\right)}}$$

配合 $$$c=21$$$，應用常數法則 $$$\int c\, dx = c x$$$：

$$\int{2 x^{2} d x} - \int{x^{3} d x} + \int{5 x^{6} d x} + {\color{red}{\int{21 d x}}} = \int{2 x^{2} d x} - \int{x^{3} d x} + \int{5 x^{6} d x} + {\color{red}{\left(21 x\right)}}$$

套用冪次法則 $$$\int x^{n}\, dx = \frac{x^{n + 1}}{n + 1}$$$ $$$\left(n \neq -1 \right)$$$，以 $$$n=3$$$：

$$21 x + \int{2 x^{2} d x} + \int{5 x^{6} d x} - {\color{red}{\int{x^{3} d x}}}=21 x + \int{2 x^{2} d x} + \int{5 x^{6} d x} - {\color{red}{\frac{x^{1 + 3}}{1 + 3}}}=21 x + \int{2 x^{2} d x} + \int{5 x^{6} d x} - {\color{red}{\left(\frac{x^{4}}{4}\right)}}$$

套用常數倍法則 $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$，使用 $$$c=2$$$ 與 $$$f{\left(x \right)} = x^{2}$$$：

$$- \frac{x^{4}}{4} + 21 x + \int{5 x^{6} d x} + {\color{red}{\int{2 x^{2} d x}}} = - \frac{x^{4}}{4} + 21 x + \int{5 x^{6} d x} + {\color{red}{\left(2 \int{x^{2} d x}\right)}}$$

套用冪次法則 $$$\int x^{n}\, dx = \frac{x^{n + 1}}{n + 1}$$$ $$$\left(n \neq -1 \right)$$$，以 $$$n=2$$$：

$$- \frac{x^{4}}{4} + 21 x + \int{5 x^{6} d x} + 2 {\color{red}{\int{x^{2} d x}}}=- \frac{x^{4}}{4} + 21 x + \int{5 x^{6} d x} + 2 {\color{red}{\frac{x^{1 + 2}}{1 + 2}}}=- \frac{x^{4}}{4} + 21 x + \int{5 x^{6} d x} + 2 {\color{red}{\left(\frac{x^{3}}{3}\right)}}$$

套用常數倍法則 $$$\int c f{\left(x \right)}\, dx = c \int f{\left(x \right)}\, dx$$$，使用 $$$c=5$$$ 與 $$$f{\left(x \right)} = x^{6}$$$：

$$- \frac{x^{4}}{4} + \frac{2 x^{3}}{3} + 21 x + {\color{red}{\int{5 x^{6} d x}}} = - \frac{x^{4}}{4} + \frac{2 x^{3}}{3} + 21 x + {\color{red}{\left(5 \int{x^{6} d x}\right)}}$$

套用冪次法則 $$$\int x^{n}\, dx = \frac{x^{n + 1}}{n + 1}$$$ $$$\left(n \neq -1 \right)$$$，以 $$$n=6$$$：

$$- \frac{x^{4}}{4} + \frac{2 x^{3}}{3} + 21 x + 5 {\color{red}{\int{x^{6} d x}}}=- \frac{x^{4}}{4} + \frac{2 x^{3}}{3} + 21 x + 5 {\color{red}{\frac{x^{1 + 6}}{1 + 6}}}=- \frac{x^{4}}{4} + \frac{2 x^{3}}{3} + 21 x + 5 {\color{red}{\left(\frac{x^{7}}{7}\right)}}$$

因此，

$$\int{\left(5 x^{6} - x^{3} + 2 x^{2} + 21\right)d x} = \frac{5 x^{7}}{7} - \frac{x^{4}}{4} + \frac{2 x^{3}}{3} + 21 x$$

化簡：

$$\int{\left(5 x^{6} - x^{3} + 2 x^{2} + 21\right)d x} = \frac{x \left(60 x^{6} - 21 x^{3} + 56 x^{2} + 1764\right)}{84}$$

加上積分常數：

$$\int{\left(5 x^{6} - x^{3} + 2 x^{2} + 21\right)d x} = \frac{x \left(60 x^{6} - 21 x^{3} + 56 x^{2} + 1764\right)}{84}+C$$

$$$\int \left(5 x^{6} - x^{3} + 2 x^{2} + 21\right)\, dx = \frac{x \left(60 x^{6} - 21 x^{3} + 56 x^{2} + 1764\right)}{84} + C$$$A