已知三点坐标，求三角形面积（转载）

//二维的情况，假设三个顶点坐标分别为(x[i], y[i]), (x[j], y[j]), (x[k], y[k])
double areaTri_2dimension(int i, int j, int k)
{
double x1 = x[j] - x[i];
double y1 = y[j] - y[i];
double x2 = x[k] - x[i];
double y2 = y[k] - y[i];
return 0.5 * fabs(x1 * y2 - y1 * x2);
}

//三维的情况，假设三个顶点坐标分别为(x[i], y[i], z[i]), (x[j], y[j], z[j]), (x[k], y[k], z[k])
double areaTri_2dimension(int i, int j, int k)
{
double x1 = x[j] - x[i];
double y1 = y[j] - y[i];
double z1 = z[j] - z[i];
double x2 = x[k] - x[i];
double y2 = y[k] - y[i];
double z2 = z[k] - z[i];
return 0.5 * sqrt(SQR(y1 * z2 - z1 * y2) + SQR(x1 * z2 - z1 * x2) + SQR(x1 * y2 - y1 * z2));
}