Go: package main

import (
        "fmt"
        "math/big"
)

func main() {
        var x1, x2, x3, x4, y1, y2, y3, y4 big.Float
        var n int
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                fmt.Scanf("%f %f %f %f %f %f %f %f",
                        &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4)
                var lx, ly, mx, my, mul1, mul2, sub, zz big.Float
                lx.Sub(&x2, &x1)
                ly.Sub(&y2, &y1)
                mx.Sub(&x4, &x3)
                my.Sub(&y4, &y3)
                mul1.Mul(&lx, &my)
                mul2.Mul(&ly, &mx)
                sub.Sub(&mul1, &mul2)
                zz.SetFloat64(0.0)

                if sub.Cmp(&zz) == 0 {
                        fmt.Println("YES")
                } else {
                        fmt.Println("NO")
                }
        }
}

C++: #ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

//#ifdef __GXX_EXPERIMENTAL_CXX0X__
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
//#endif

#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#include <array>
//#include <atomic>
#include <chrono>
//#include <condition_variable>
#include <forward_list>
//#include <future>
#include <initializer_list>
//#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <system_error>
//#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>


using namespace std;


class D3
{
public:
	double x;
	double y;
	double z;

	//?????????????????????
	D3 ( double x_ = 0.0 , double y_ = 0.0 , double z_ = 0.0 )
	{
		x = x_;
		y = y_;
		z = z_;
	}

	//??????
	D3 Cross_product ( D3 partner )
	{
		D3 ret ( y*partner.z - z*partner.y , z*partner.x - x*partner.z , x*partner.y - y*partner.x );
		return ret;
	}

	//??????
	double Inner_product ( D3 partner )
	{
		return x*partner.x + y*partner.y + z*partner.z;
	}

	//?????????????????????????????????
	D3 Make_vector ( D3 partner )
	{
		D3 ret ( partner.x - x , partner.y - y , partner.z - z );
		return ret;
	}

	//???????§????2????????????????????¢???????±???????
	double S_vec ( D3 A , D3 B )
	{
		D3 G;
		G = A.Cross_product ( B );
		return sqrt ( G.Inner_product ( G ) ) / 2;
	}

	//??????????????¢???????±???????
	double S_point ( D3 A , D3 B , D3 C )
	{
		return S_vec ( A.Make_vector ( B ) , A.Make_vector ( C ) );
	}

	//???????§????3????????????????????????????±???????
	double V_vec ( D3 A , D3 B , D3 C )
	{
		D3 G;
		G = A.Cross_product ( B );
		return G.Inner_product ( C ) / 6;
	}

	//4???????????????????±???????
	double V_point ( D3 A , D3 B , D3 C , D3 D )
	{
		return V_vec ( A.Make_vector ( B ) , A.Make_vector ( C ) , A.Make_vector ( D ) );
	}

	D3 operator+( D3 partner )
	{
		return D3 ( x + partner.x , y + partner.y , z + partner.z );
	}

	D3 operator-( D3 partner )
	{
		return D3 ( x - partner.x , y - partner.y , z - partner.z );
	}

	bool operator==( D3 partner )
	{
		return x == partner.x&&y == partner.y&&z == partner.z;
	}

	double length ()
	{
		return sqrt ( x*x + y*y + z*z );
	}

	//?????´????????????
	bool Vertical ( D3 partner )
	{
		return ( *this ).Inner_product ( partner ) == 0;
	}

	//??????????????????
	bool Parallel ( D3 partner )
	{
		return abs(abs ( ( *this ).Inner_product ( partner ) ) - abs ( ( *this ).length ()*partner.length () ))-1e-3<0;
	}
};

int main ()
{
	double a[8];
	int N;
	cin >> N;
	for( size_t k = 0; k < N; k++ )
	{
		for( size_t i = 0; i < 8; i++ )
		{
			cin >> a[i];
		}
		D3 A ( a[0] , a[1] ) , B ( a[2] , a[3] ) , C ( a[4] , a[5] ) , D ( a[6] , a[7] );
		D3 AB = B - A , CD = D - C;
		cout << ( AB.Parallel ( CD ) ? "YES" : "NO" ) << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;

int main() {
	double x1, y1, x2, y2, x3, y3, x4, y4;
	int n;
	double veku1[2];
	double veku2[2];
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		veku1[0] = x2 - x1;
		veku1[1] = y2 - y1;
		veku2[0] = x4 - x3;
		veku2[1] = y4 - y3;
		if (fabs(veku1[0]*veku2[1]-veku1[1]*veku2[0]) <= 0.0000001) {
			/*veku1[0] = x3 - x1;
			veku1[1] = y3 - y1;
			veku2[0] = x4 - x1;
			veku2[1] = y4 - y1;
			if (fabs(veku1[0]*veku2[1]-veku1[1]*veku2[0]) <= 0.0000001) {
				cout << "NO" << endl;
			} else {*/
				cout << "YES" << endl;
			//}
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
 
using namespace std;

#define EPS 1e-5
 
struct point{
	double x;
	double y;
};

struct line{
	double a;
	double b;
};

bool get_line( const point& a , const point& b , line *a_b );
istream& operator >> ( istream& s , point& p );
 
int main(void)
{
	point a, b, c, d;
 
    line a_b, c_d;
 
    bool result1 , result2;
 
    int num;
 
    cin >> num;
 
    for( int i = 0 ; i < num ; i++ )
    {
        cin >> a >> b >> c >> d;
 
        result1 = get_line( a , b , &a_b );
        result2 = get_line( c , d , &c_d );
 
        if( result1 == false && result2 == false )
        {
            cout << "YES" << endl;
        }
        else
        {
            if( fabs(a_b.a - c_d.a) < EPS ) cout << "YES" << endl;
            else cout << "NO" << endl;
        }
    }
 
    return 0;
}

istream& operator >> ( istream& s , point& p )
{
	return s >> p.x >> p.y;
}

bool get_line( const point& a , const point& b , line *a_b )
{
	if( fabs(a.x - b.x) < EPS ) return false;
	else{
		a_b->a = (a.y - b.y)/(a.x - b.x);
		a_b->b = a.y - (a.x * a_b->a);
		return true;
	}
}

C++: #include<iostream>
#include<math.h>
using namespace std;

int main()
{
    int n;
    cin>>n;
    while(n--){
        pair<double,double>a[4];
        for(int i=0;i<4;i++){
            cin>>a[i].first>>a[i].second;
        }
        if(abs((a[1].first-a[0].first)*(a[3].second-a[2].second)-(a[3].first-a[2].first)*(a[1].second-a[0].second))<0)
            cout<<"YES"<<endl;
        else
            cout<<"NO"<<endl;
    }
    return 0;
}

C++: #include <iostream>
using namespace std;

int main() {
	int N;
    double x1,y1,x2,y2,x3,y3,x4,y4;
    cin >> N;
    for (int n = 0; n != N; ++n) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        if ((y2-y1)/(x2-x1) == (y4-y3)/(x4-x3)) {
            cout << "YES" << endl;
        }
        else {
            cout << "NO" << endl;
        }
    }
    return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
  int n;
  double xa,ya,xb,yb,xc,yc,xd,yd,a2,a1;
  while(cin>>n){
    for(int i=0;i<n;i++){
      cin>>xa>>ya>>xb>>yb>>xc>>yc>>xd>>yd;
      a1=(yb-ya)/(xb-xa);
      a2=(yd-yc)/(xd-xc);
      if(a1==a2)cout<<"YES"<<endl;
      else cout<<"NO"<<endl;
    }
  }
  return 0;
}