三角形面积重要公式

a = sqrt((x1 - x2)*(x1 - x2) + (y1 - y2)*(y1 - y2));
b = sqrt((x1 - x3)*(x1 - x3) + (y1 - y3)*(y1 - y3));
c = sqrt((x2 - x3)*(x2 - x3) + (y2 - y3)*(y2 - y3));
p = (a + b + c) / 2;
s = sqrt(p * (p - a) * (p - b) * (p - c));