$$$x^{2} + 6 x + 25$$$ 的導數

和/差的導數等於導數的和/差：

$${\color{red}\left(\frac{d}{dx} \left(x^{2} + 6 x + 25\right)\right)} = {\color{red}\left(\frac{d}{dx} \left(x^{2}\right) + \frac{d}{dx} \left(6 x\right) + \frac{d}{dx} \left(25\right)\right)}$$

常數的導數為$$$0$$$：

$${\color{red}\left(\frac{d}{dx} \left(25\right)\right)} + \frac{d}{dx} \left(6 x\right) + \frac{d}{dx} \left(x^{2}\right) = {\color{red}\left(0\right)} + \frac{d}{dx} \left(6 x\right) + \frac{d}{dx} \left(x^{2}\right)$$

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

$${\color{red}\left(\frac{d}{dx} \left(6 x\right)\right)} + \frac{d}{dx} \left(x^{2}\right) = {\color{red}\left(6 \frac{d}{dx} \left(x\right)\right)} + \frac{d}{dx} \left(x^{2}\right)$$

套用冪次法則 $$$\frac{d}{dx} \left(x^{n}\right) = n x^{n - 1}$$$，取 $$$n = 1$$$，也就是 $$$\frac{d}{dx} \left(x\right) = 1$$$：

$$6 {\color{red}\left(\frac{d}{dx} \left(x\right)\right)} + \frac{d}{dx} \left(x^{2}\right) = 6 {\color{red}\left(1\right)} + \frac{d}{dx} \left(x^{2}\right)$$

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

$${\color{red}\left(\frac{d}{dx} \left(x^{2}\right)\right)} + 6 = {\color{red}\left(2 x\right)} + 6$$

因此，$$$\frac{d}{dx} \left(x^{2} + 6 x + 25\right) = 2 x + 6$$$。