求高手帮看程序错误在哪??有关c++中的函数声明定义与调用

程序如下,总是报错...报错内容在图里...似乎说是类型不符合/...
但是我也不知道为啥不符合,,,
学c++一个学期以后又过了一年多了,真是不记得啥了= =`查书又查不到555...
小女子先行谢过了!!!

#include <iostream>

#include<cmath>

using namespace std;

double Sum(double,double,int);

int main()

{

double a[6][6]={ 4,-1, 0,-1, 0, 0,

-1, 4,-1, 0,-1, 0,

0,-1, 4, 0, 0,-1,

-1, 0, 0, 4,-1, 0,

0,-1, 0,-1, 4,-1,

0, 0,-1, 0,-1, 4},

b[6]={2,1,2,2,1,2},

xx[6]={0.5,0.5,0.5,0.5,0.5,0.5},

x[6];

int i,t;

cout << "使用的初始x0各个分量为 0.5 " << endl << endl;

for (t=1; ;t++ )

{

for (i=0;i<6;i++)

{

x[i]= (b[i]-Sum(a,xx,i)/a[i][i];

xx[i]=x[i];

sum=0;

}

if (fabs(xx[i]-x[i])<10e-6) break;

if (t>40) {cout << endl << "error!" << endl; return 0;}

}

for (i=0; i<6; i++)

{

cout << "经过了" << t << "次迭代以后¸可以得到x的第" << i+1 << "个分量为" << x[i] << endl;

}

return 0;

}

double Sum(double a[][],double xx[],int i)

{

double sum=0;

int j=0;

for (;j<i;j++)

sum = sum+a[i][j]*xx[j];

for (;j<6;j++)

sum = sum+a[i][j]*xx[j];

return sum;

}
报错如下: