#include<stdio.h>
#include<math.h>
float distance(int x1,int x2,int y1,int y2)
{
float distance;
distance=sqrt(((x1-x2)*(x1-x2))+((y1-y2)*(y1-y2)));
return distance;
}
int main()
{
int x1,x2,x3,y1,y2,y3;
float a1,a2,a3;
printf("Enter the x-coordinate of vertex A\n");
scanf("%d",&x1);
printf("Enter the y-coordinate of vertex A\n");
scanf("%d",&y1);
printf("Enter the x-coordinate of vertex B\n");
scanf("%d",&x2);
printf("Enter the y-coordinate of vertex B\n");
scanf("%d",&y2);
printf("Enter the x-coordinate of vertex C\n");
scanf("%d",&x3);
printf("Enter the y-coordinate of vertex C\n");
scanf("%d",&y3);
a1=distance(x1,x2,y1,y2);
a2=distance(x2,x3,y2,y3);
a3=distance(x1,x3,y1,y3);
printf("Length of side AB is %.1f\n",a1);
printf("Length of side BC is %.1f\n",a2);
printf("Length of side AC is %.1f\n",a3);
return 0;
}

*Related*

C