Apothem

Apothem is the length of a line perpendicular from the center to the midpoint of the side of a regular polygon.
A irregular polygon has no center so there can be no apothem.

Apothem Edge formula
$\large{A=\frac {s} {2 \;tan\; \left( \frac{180}{n} \right) } }$
Symbol	English	Metric
$\large{A}$ = apothem	$\large{in}$	$\large{mm}$
$\large{s}$ = edge (side length)	$\large{in}$	$\large{mm}$
$\large{n}$ = number of sides	$\large{dimensionless}$
$\large{tan}$ = tangent	$\large{deg}$	$\large{rad}$

Apothem Edge formula