3 is known triangle vertex coordinates A (x1, y1), B (x2, y2), C (x3, y3), the area of triangle ABC find the formula
S=1/2[(x1y2-x2y1)+(x2y3-x3y2)+(x3y1-x1y3)]
Suppose in a plane, there is a triangle with sides respectively A, B, C of the triangle area S is obtained by the following formula:
= √ S [p (PA) (Pb) (p- C)] of the formula where p is semigirth: p = (A + B + C) / 2