Need help with function prototyping

[dcabbil]

How do I get my program to work using the void function prototype?

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

void get_Angles(double a, double b, double c, double& angle_a, double& angle_b, double& angle_c);
int main()
{
double s, area, AngleA, AngleB, AngleC, in_radius, out_radius, sum, perimeter, diff_radii, lower_a, lower_b, lower_c, upper_a , upper_b, upper_c, step_a, step_b, step_c;

ofstream fout("triangles.dat");

cout << "Enter in your integers for your triangle: \n";
cin >> lower_a >> lower_b >> lower_c >> upper_a >> upper_b >> upper_c >> step_a >> step_b >> step_c;

for(double a=lower_a; a<=upper_a; a+=step_a){
for(double b=lower_b; b<=upper_b; b+=step_b){
for(double c=lower_c; c<=upper_c; c+=step_c){

area=sqrt(s*(s-a)*(s-b)*(s-c));
if(area >=0)
{
perimeter=a+b+c;
s=(a+b+c)/2.0;
double angle_a=2.0*acos(sqrt((s*(s-a))/(b*c)));
AngleA=(angle_a*180.0)/3.14;
double angle_b=2.0*acos(sqrt((s*(s-b))/(a*c)));
AngleB=(angle_b*180.0)/3.14;
double angle_c=2.0*acos(sqrt((s*(s-c))/(a*b)));
AngleC=(angle_c*180.0)/3.14;
in_radius=area/s;
out_radius=(a*b*c)/(4*area);
sum=angle_a+angle_b+angle_c;
diff_radii=out_radius-in_radius;

cout << "Perimeter=" << perimeter << endl
<< "S=" << s << endl
<< "Area=" << area << endl
<< "Angle A=" << AngleA << endl
<< "Angle B=" << AngleB << endl
<< "Angle C=" << AngleC << endl
<< "Sum of Angles=" << sum << endl
<< "Inner Radius=" << in_radius << endl
<< "Outer Radius=" << out_radius << endl
<< "Difference in Radii=" << diff_radii << endl;
}
else
cout << "The numbers that you have inputted is not considered a triangle.\n";
}
}
}

return 0;
}