7.1

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