果然是，谢谢gbgame了。

#include <stdio.h>
#include <math.h>
#define S(x,y,z) (a+b+c)/2
#define A(x,y,z) sqrt(S(x,y,z)*(S(x,y,z)-x)*(S(x,y,z)-y)*(S(x,y,z)-z))
#include <conio.h>

int main()
{
double a,b,c;
do
{
printf("Input a、b、c：\n");
scanf("%lf %lf %lf",&a,&b,&c);
}while((a+b)<c||(a+c)<b||(b+c)<a);
printf("%.2lf\n",A(a,b,c));
getch();
}