Approximation error

In mathematics, the error of the numerical value of b as an approximation of a is:

${\displaystyle a-b\,\!.}$

The absolute error of b as an approximation of a is:

${\displaystyle |a-b|\,\!.}$

The relative error of b as an approximation of a is:

${\displaystyle {\frac {|a-b|}{|a|}}.}$