（递归）运行有警告，结果错误，怎么回事？

#include<stdio.h>
#define N 10
void main()
{
double x(int n);
double s(int n);
double n,sum;
sum=s(n);
printf("前10项和为: %lf\n ",sum);
printf("\n");
}
double s(int n)
{
double x(int n);
double i,sum=3;
for(i=3;i<=10;i++)
sum+=x(n);
return sum;
}
double x(int n)
{
double x1=1,x2=2,x3;
if(n=1)
return x3=1;
else if(n=2)
return x3=2;
else
{   x(n);
x3=x1*x2;
return x(n--);
}
return x3;
}