for p1, p2, p3 being Point of (TOP-REAL 2)
for a, b, c, s being Real st a = |.(p2 - p1).| & b = |.(p3 - p2).| & c = |.(p1 - p3).| & s = (the_perimeter_of_polygon3 (p1,p2,p3)) / 2 holds
|.(the_area_of_polygon3 (p1,p2,p3)).| = sqrt (((s * (s - a)) * (s - b)) * (s - c))