Derivative of $$$x^{3} - 2 x$$$ at $$$x = c$$$
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Your Input
Find $$$\frac{d}{dx} \left(x^{3} - 2 x\right)$$$ and evaluate it at $$$x = c$$$.
Solution
The derivative of a sum/difference is the sum/difference of derivatives:
$${\color{red}\left(\frac{d}{dx} \left(x^{3} - 2 x\right)\right)} = {\color{red}\left(\frac{d}{dx} \left(x^{3}\right) - \frac{d}{dx} \left(2 x\right)\right)}$$Apply the power rule $$$\frac{d}{dx} \left(x^{n}\right) = n x^{n - 1}$$$ with $$$n = 3$$$:
$${\color{red}\left(\frac{d}{dx} \left(x^{3}\right)\right)} - \frac{d}{dx} \left(2 x\right) = {\color{red}\left(3 x^{2}\right)} - \frac{d}{dx} \left(2 x\right)$$Apply the constant multiple rule $$$\frac{d}{dx} \left(c f{\left(x \right)}\right) = c \frac{d}{dx} \left(f{\left(x \right)}\right)$$$ with $$$c = 2$$$ and $$$f{\left(x \right)} = x$$$:
$$3 x^{2} - {\color{red}\left(\frac{d}{dx} \left(2 x\right)\right)} = 3 x^{2} - {\color{red}\left(2 \frac{d}{dx} \left(x\right)\right)}$$Apply the power rule $$$\frac{d}{dx} \left(x^{n}\right) = n x^{n - 1}$$$ with $$$n = 1$$$, in other words, $$$\frac{d}{dx} \left(x\right) = 1$$$:
$$3 x^{2} - 2 {\color{red}\left(\frac{d}{dx} \left(x\right)\right)} = 3 x^{2} - 2 {\color{red}\left(1\right)}$$Thus, $$$\frac{d}{dx} \left(x^{3} - 2 x\right) = 3 x^{2} - 2$$$.
Finally, evaluate the derivative at $$$x = c$$$.
$$$\left(\frac{d}{dx} \left(x^{3} - 2 x\right)\right)|_{\left(x = c\right)} = 3 c^{2} - 2$$$
Answer
$$$\frac{d}{dx} \left(x^{3} - 2 x\right) = 3 x^{2} - 2$$$A
$$$\left(\frac{d}{dx} \left(x^{3} - 2 x\right)\right)|_{\left(x = c\right)} = 3 c^{2} - 2$$$A