Почему мы пишем double x (double y)? а не через запятую double x,y

#include <vcl.h>
#pragma hdrstop
#include <iostream.h>
#include <math.h>
#include<conio.h>
using namespace std;
#pragma argsused
double Y(double x)
{
return exp(x)+exp(-x)/2;
}
 
double S(double x, long n)
{
double ak = 1;
double sum = 0;
for(long k = 0; k < n; k++)
{
 
sum += ak;
ak  *= pow(x,2*k)/(1.0*(2*k + 1)*(k + 1));
}
return sum;
}
int main()
{
double xn = 0, xk = 0, hx = 0;
double y, s, x;
long n;
printf("Enter xn: ");scanf("%lf",&xn);
printf("Enter xk: ");scanf("%lf",&xk);
printf("Enter hx: ");scanf("%lf",&hx);
printf("Enter n : ");scanf("%u",&n);
printf("----------------------------------\n");
printf("  Y(x)  |  S(x)  |  |Y(x)-S(x)|  |\n");
printf("----------------------------------\n");
for(x = xn; x <= xk + hx; x += hx)
{
y = Y(x);
s = S(x,n);
printf("  %.3lf  |  %.3lf  |    %.3lf \n",
Y(x),S(x,n),fabs(y-s));
}
printf("Press any key to continue\n");
getch();
return 0;
}

sum += ak;
ak  *= pow(x,2*k)/(1.0*(2*k + 1)*(k + 1));
}
return sum;

# include <iostream>
 
using namespace std;
int main()
{
    double x(double y);
    x(100);
 
}
 
double x(double y){
    cout<<y<<"\n";
    return 0;
}

sum += ak;
ak  *= pow(x,2*k)/(1.0*(2*k + 1)*(k + 1));
}
return sum;

double ak = 1;
double sum = 0;
for(long k = 0; k < n; k++)