作業題：求三角形的面積

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main(){
    double a,b,c;
    cout<<"please enter a,b,c:";
    cin>>a>>b>>c;
    if(a+b>c&&b+c>a&&c+a>b){
        double s,area;
        s=(a+b+c)/2;
        area=sqrt(s*(s-a)*(s-b)*(s-c));
        cout<<setiosflags(ios::fixed)<<setprecision(4);//就是保留4位小數
        cout<<"area="<<area<<endl;
    }else{
        cout<<"it is not a trilateral!"<<endl;
    
    }
    
    

    
    return 0;
}