Inscribed equilateral triangle:

Draw apothem from center perpendicular to a side.

Draw radius from center to vertex of same side.

Triangle formed is 30-60-90.

If hypotenuse(radius) is 8, then side opposite the 30 deg. angle is 4 (apothem).

Side opposite 60 degree angle is

Thus, the length of a side is

Perimeter =

, where P=perimeter, a=apothem

Finally,