Derivative of $$$e^{- x^{2}}$$$
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Solution
The function $$$e^{- x^{2}}$$$ is the composition $$$f{\left(g{\left(x \right)} \right)}$$$ of two functions $$$f{\left(u \right)} = e^{u}$$$ and $$$g{\left(x \right)} = - x^{2}$$$.
Apply the chain rule $$$\frac{d}{dx} \left(f{\left(g{\left(x \right)} \right)}\right) = \frac{d}{du} \left(f{\left(u \right)}\right) \frac{d}{dx} \left(g{\left(x \right)}\right)$$$:
$${\color{red}\left(\frac{d}{dx} \left(e^{- x^{2}}\right)\right)} = {\color{red}\left(\frac{d}{du} \left(e^{u}\right) \frac{d}{dx} \left(- x^{2}\right)\right)}$$The derivative of the exponential is $$$\frac{d}{du} \left(e^{u}\right) = e^{u}$$$:
$${\color{red}\left(\frac{d}{du} \left(e^{u}\right)\right)} \frac{d}{dx} \left(- x^{2}\right) = {\color{red}\left(e^{u}\right)} \frac{d}{dx} \left(- x^{2}\right)$$Return to the old variable:
$$e^{{\color{red}\left(u\right)}} \frac{d}{dx} \left(- x^{2}\right) = e^{{\color{red}\left(- x^{2}\right)}} \frac{d}{dx} \left(- x^{2}\right)$$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 $$$c = -1$$$ and $$$f{\left(x \right)} = x^{2}$$$:
$$e^{- x^{2}} {\color{red}\left(\frac{d}{dx} \left(- x^{2}\right)\right)} = e^{- x^{2}} {\color{red}\left(- \frac{d}{dx} \left(x^{2}\right)\right)}$$Apply the power rule $$$\frac{d}{dx} \left(x^{n}\right) = n x^{n - 1}$$$ with $$$n = 2$$$:
$$- e^{- x^{2}} {\color{red}\left(\frac{d}{dx} \left(x^{2}\right)\right)} = - e^{- x^{2}} {\color{red}\left(2 x\right)}$$Thus, $$$\frac{d}{dx} \left(e^{- x^{2}}\right) = - 2 x e^{- x^{2}}$$$.
Answer
$$$\frac{d}{dx} \left(e^{- x^{2}}\right) = - 2 x e^{- x^{2}}$$$A