void chobsb(double a[], int n, double d[], double b[])
{
	int i,j;
	double sum;
    for (i = 1; i<=n; i++)
	{
        sum = b[i];
        for (j = 1; j<=i - 1; j++)
		{
            sum = sum - a[(i-1)*n+j] * b[j];
        }
        b[i] = sum;
    }
    for (i = n; i>=1; i--)
	{
        if (d[i] == 0.0)
		{
            cout<<"  singular matrix";
            return;
		}
        else
		{
            sum = b[i] / d[i];
        }
        for (j = i + 1; j<=n; j++)
		{
            sum = sum - a[(j-1)*n+i] * b[j];
        }
        b[i] = sum;
    }
}