三点坐标求三角形面积（数学）

struct point{double x, y;}p[105];

double xmult(point p1,point p2,point p0)
{
    return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}

double area_triangle(point p1,point p2,point p3)
{
    return fabs(xmult(p1,p2,p3))/2;
}

for(int i = 0; i < m; i++)
        {
            for(int j = 0; j < m; j++)
            {
                for(int k = 0; k < m; k++)
                {
                    S = area_triangle(p[i],p[j],p[k]);
                }
            }
        }