fractional uncertainty

$x_{best}=x_{best}\left(1\pm \frac{\delta x}{|x_{best}|}\right)$

$x_{best}$ is the value obtained from an experiment, $\delta x$ is the uncertainty/error of $x$. In multiplication, uncertainty is measured using percentages to make multiplication easier.

Example

$q=xy$

$q_{best}=x_{best}\left(1+\frac{\delta x}{|x_{best}|}\right)y_{best}\left(1+\frac{\delta y}{|y_{best}|}\right)$

$q_{best}=x_{best}\cdot y_{best}\left(1+\left(\frac{\delta x}{|x_{best}|}+\frac{\delta y}{|y_{best}|}\right)\right)$ Note that the term $\frac{\delta x\delta y}{|x_{best}||y_{best}|}$ is omitted since it is too insignificant in well-designed experiments.