```//* Three purposes of using functions *
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <math.h>

double Distance_Between_Points(double point1x, double point1y, double point2x, double point2y);
double Area_of_Triangle(double leg_lenght_a, double leg_lenght_b, double leg_lenght_c);

int main() {
double point1x, point1y, point2x, point2y, point3x, point3y;
double d1, d2, d3, area;

printf("Please enter the coordinates of the triangle vertices and press ENTER.\n");
scanf("%lf %lf %lf %lf %lf %lf", &point1x, &point1y, &point2x, &point2y, &point3x, &point3y);
d1=Distance_Between_Points(point1x, point1y, point2x, point2y);
d2=Distance_Between_Points(point3x, point3y, point2x, point2y);
d3=Distance_Between_Points(point1x, point1y, point3x, point3y);
area=Area_of_Triangle(d1, d2, d3);
printf("The area of the triangle is %.2lf\n",  area);
return(0);
}

/* inline */ double sqr (double x) {
// return(pow(fabs(x),2));
return(x*x);
}

double Distance_Between_Points(double x1, double y1, double x2, double y2) {
return( sqrt( sqr(x2-x1) + sqr(y2-y1) ) );
// OR
// return( sqrt( pow( fabs(x2-x1),2)+pow( fabs(y2-y1),2) ) );
// warning: C library requires that the base in pow is >0 !!!
}

double Area_of_Triangle(double leg_lenght_a, double leg_lenght_b, double leg_lenght_c) {
// Hero's formula for the triangle area
double part=(leg_lenght_a+leg_lenght_b+leg_lenght_c)/2.0;
double area=sqrt(part*(part-leg_lenght_a)*(part-leg_lenght_b)*(part-leg_lenght_c));
return(area);
}

```