Elemento de distância

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$\displaystyle \vert d\vec{r}\vert$	$\textstyle =$	$\displaystyle \sqrt{ \left( \frac{dx}{du} \right)^2 du^2 + \left( \frac{dy}{du} \right)^2 du^2 + \left( \frac{dz}{du} \right)^2 du^2 }$
	$\textstyle =$	$\displaystyle \sqrt{ \left( \frac{dx}{du} \right)^2 + \left( \frac{dy}{du} \right)^2 + \left( \frac{dz}{du} \right)^2 } du$