#include<iostream.h>
#include<math.h>

struct Tangle
{ public: double a,b,c;
double area()
{double s,ar;
s=(a+b+c)/2;
ar=sqrt((s-a)*(s-b)*(s-c));
return ar;
}
}

void main()
{ Tangle t;
cout<<endl<<"a=";
cin>>t.a;
cout<<endl<<"b=";
cin>>t.b;
cout<<endl<<"c=";
cin>>t.c;
cout<<"The Area is "<<t.area.ar;
}

#include<iostream.h>
#include<math.h>
class Tangle
{
public: double a,b,c;
double area()
{double s,ar;
s=(a+b+c)/2;
ar=sqrt((s-a)*(s-b)*(s-c));
return ar;
}
};

void main()
{ Tangle t;
cout<<endl<<"a=";
cin>>t.a;
cout<<endl<<"b=";
cin>>t.b;
cout<<endl<<"c=";
cin>>t.c;
cout<<"The area is"<<t.area();
}
就改了下语法错误，其他没看；

如果我要用 struct 怎么写呢

把class改成struct就行了

class 和 struct 用法是一样的，区别在于stuct 的成员默认是public,而class 成员默认是private;