3.1

double distance(double x1, double y1, double x2, double y2) {
    return sqrt(pow(x2 - x1, 2) + pow(y2 - y1, 2));
}

double area(double x1, double y1, double x2, double y2, double x3, double y3) {
    double a = distance(x1, y1, x2, y2);
    double b = distance(x1, y1, x3, y3);
    double c = distance(x2, y2, x3, y3);
    double s = (a + b + c) / 2;
    return sqrt(s*(s - a)*(s - b)*(s - c));
}

int main() {
    double x1, y1, x2, y2, x3, y3;
    cout << "TRIANGLE AREA CALCULATOR" << endl;
    cout << "V1 (x1 y1) ? ";
    cin >> x1 >> y1;
    cout << "V2 (x2 y2) ? ";
    cin >> x2 >> y2;
    cout << "V3 (x3 y3) ? ";
    cin >> x3 >> y3;
    cout << fixed << setprecision(2) << "Area = " << area(x1, x2, x2, y2, x3, y3) << endl;
    return 0;
}