各位大神帮我找找错误

file1.cpp(文件1)
#include<iostream>
using namespace std;
int main()
{
  extern double a,b,c;
  cin>>a>>b>>c;
  cout<<length(a,b,c)<<endl;
  return 0;
}
file2.cpp(文件2)
double is Triangle(double a,doublet b,double c)
x=a+b;
y=b+c;
z=a+c;
{
    if(x>c,y>a,z>b) return true
        else return false;
file3.cpp(文件3)
  double length(double a,doublet b,double c)
    {double p
     p=a+b+c;
    return (p);
    }