高斯消元

double a[maxn][maxn],x[maxn];
void gauss()
{
	for1(i,n-1)
	{
		int k=i;
		for2(j,i+1,n)if(fabs(a[j][i])>fabs(a[k][i]))k=j;
		for2(j,i,n+1)swap(a[i][j],a[k][j]);
		for2(j,i+1,n)
		{
			double tmp=a[j][i]/a[i][i];
			for2(k,i,n+1)a[j][k]=a[i][k]*tmp-a[j][k];
		}
	}
	x[n]=a[n][n+1]/a[n][n];
	for3(i,n-1,1)
	{
		double tmp=0;
		for2(j,i+1,n)tmp+=x[j]*a[i][j];
	    x[i]=(a[i][n+1]-tmp)/a[i][i];
	}
}