7-1

#include <iostream>
#include <cmath>
using namespace std;
void area(float a,float b,float c)
{
 double s,area;
 if(a+b<=c)
	 cerr<<"a+b<=c,error!"<<endl;
 else if(b+c<=a)
	 cerr<<"b+c<=a,error!"<<endl;
 else if(a+c<=b)
	 cerr<<"a+c<=b,error!"<<endl;
 else
 {
  s=(a+b+c)/2;
  area=sqrt(s*(s-a)*(s-b)*(s-c));
  cout<<"area="<<area<<endl;
 }
}
int main()
{
 float a=2,b=3,c=4;
 area(a,b,c);
 return 0;
}