09/04/16 14:04:39
#include <stdio.h>
#include <math.h>
int main(void)
{
double x1, y1, x2, y2, x3, y3;
double a, b, c, s, S;
#define INPUT(x) printf(#x ": "); scanf("%lf", &x)
INPUT(x1); INPUT(y1);
INPUT(x2); INPUT(y2);
INPUT(x3); INPUT(y3);
a = sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
b = sqrt((x3 - x2) * (x3 - x2) + (y3 - y2) * (y3 - y2));
c = sqrt((x1 - x3) * (x1 - x3) + (y1 - y3) * (y1 - y3));
s = (a + b + c) / 2.0;
S = sqrt(s * (s - a) * (s - b) * (s - c));
printf("三角形の面積は %lg\n", S);
return 0;
}