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

int main(){
    const double eps = 1e-11;
    double x[4], y[4];
    int s,t,u,v;
    int n;
    cin >> n;
    for (int t = 0;t < n;++t) {
        for (int i = 0;i < 4;++i){
            cin >> x[i] >> y[i]; 
        }
    }
    s = x[1] - x[0];
    t = y[1] - y[0];
    u = x[3] - x[2];
    v = y[3] - y[2];
    if(s * v - t * u > 0){
        int m = s * v - t * u;
    }
    else{
        int m = t * u - s * v;
    }
    if( m < eps){
        cout << "YES" << endl;
    }
    else{
        cout << "NO" << endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

int main() {
  int n; cin >> n;
  for ( int i = 0; i < n; i++ ) {
    double x1,y1,x2,y2,x3,y3,x4,y4; 
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    double res1 = (x1 - x2)*(x3 - x4);
    double res2 = (y1 - y2)*(y3 - y4);

    if ( res1 == res2 ) cout << "YES" << endl;
    else cout << "NO" << endl;
  }

  return 0;
}

C++: #include <iostream>

using namespace std;

int main() {
  int n, i;
  double x1, y1, x2, y2, x3, y3, x4, y4;

  cin >> n;

  for (i = 0; i < n; i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

    if ((y2 - y1) / (x2 - x1) == (y3 - y4) / (x3 - x4)) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }

  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for(int i = s; i < (int)n; i++)
#define per(i, n) for(int i = n; i >= 0; i--)
#define ROF(i, s, n) for(int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\n"
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define EACH(i, n) for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define CLR(a) memset((a), 0 ,sizeof(a))
#define dump(x) cout << #x << " = " << (x) << "\n";
#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << "\n";
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue< int, vector<int>, greater<int> > minpq;
static const double EPS = 1e-10;
static const double PI = acos( -1.0 );
static const int mod = 10007;
static const int INF = 1 << 25;
static const LL LL_INF = 1152921504606846976;
static const int dx[] = { -1, 0, 1, 0,  1, -1,  1, -1 };
static const int dy[] = { 0, -1, 0, 1 , 1,  1, -1, -1 };

int n;
double x1, x2, x3, x4, y, y2, y3, y4;

int main() {
	scanf( "%d", &n );
	for ( int i = 0; i < n; i++ ) {
		scanf( "%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y, &x2, &y2, &x3, &y3, &x4, &y4 );
		if ( x2 - x1 == x4 - x3 && y2 - y == y4 - y3 ) {
			printf( "YES\n" );
		}
		else {
			printf( "NO\n" );
		}
	}
	return 0;
}

C++: #define _USE_MATH_DEFINES
#include<iostream>
#include<iomanip>
#include<algorithm>
#include<cctype>
#include<cmath>
#include<cstdio>
#include<deque>
#include<list>
#include<set>
#include<stack>
#include<string>
#include<sstream>
#include<map>
#include<queue>
#include<vector>
using namespace std;

int main() {
	int n;
	cin >> n;
	while (n--) {
		double x1, y1, x2, y2, x3, y3, x4, y4;
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		double ABx = x2 - x1,
			ABy = y2 - y1,
			CDx = x4 - x3,
			CDy = y4 - y3;
		if (ABy / ABx == CDy / CDx)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}

C++: #include<iostream>
int main()
{
	int n;
	std::cin >> n;
	for (int i = 0; i < n; i++)
	{
		double x1, y1, x2, y2, x3, y3, x4, y4;
		std::cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		std::cout << (x1 == x2 && x3 == x4 || (y1 - y2) / (x1 - x2) == (y3 - y4) / (x3 - x4) ? "YES" : "NO") << std::endl;
	}
	return 0;
}

C++: #include<iostream>

using namespace std;

bool get_line( double , double , double , double , double& , double& );

int main(void)
{
	double x1, x2, x3, x4;
	double y1, y2, y3, y4;

	double a1, a2;
	double b1, b2;

	bool result1 , result2;

	int num;

	cin >> num;

	for( int i = 0 ; i < num ; i++ )
	{
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

		result1 = get_line( x1 , x2 , y1 , y2 , a1 , b1 );
		result2 = get_line( x3 , x4 , y3 , y4 , a2 , b2 );

		if( result1 == false && result2 == false ) cout << "YES" << endl;

		else
		{
			if( a1 == a2 ) cout << "YES" << endl;
			else cout << "NO" << endl;
		}
	}

	return 0;
}

bool get_line( double x1 , double x2 , double y1 , double y2 , double& a , double& b )
{
	// if( x1 - x2 >= -0.0000001 && x1 - x2 <= 0.0000001 ) return false;

	if( x1 == x2 ) return false;
	else
	{
		a = ( y1 - y2 ) / ( x1 - x2 );
		b = y1 - a * x1;
		return true;
	}
}

C++: #include <stdio.h>
#include <cctype>
#include <limits.h>
#include <math.h>
#include <complex>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <cstring>
#include <string>
#include <sstream>
#include <algorithm>
#include <iomanip>
#include <iostream>

#define VARIABLE(x) cerr << #x << "=" << x << endl
#define BINARY(x) static_cast<bitset<16> >(x)
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define REP(i,m,n) for (int i=m;i<(int)(n);i++)
#define if_range(x, y, w, h) if (0<=(int)(x) && (int)(x)<(int)(w) && 0<=(int)(y) && (int)(y)<(int)(h))
#define ALL(a) (a).begin(),(a).end()
const int INF = 1e9;
const double PI = 3.14159;
int dx[4]={0, 1, 0, -1}, dy[4]={-1, 0, 1, 0};
using namespace std;
typedef long long ll;
//typedef pair<int, int> P;

/* struct P {
	int x, y, n;
	P(int n, int x, int y):n(n), x(x), y(y){}
	P(){}
}; */


/** 幾何ライブラリ **/
// to do : 1183

#include <complex>

#define X real()
#define Y imag()

const double EPS = 1e-11;

typedef double D;
typedef complex<D> P; // Point

// 共役複素数 conj(a);

// 符号
int sig(D a, D b=0) { return a < b - EPS ? -1 : (a > b + EPS ? 1 : 0); }

// 内積
D dot_product(P a, P b) { return (conj(a)*b).real(); }

// 外積
D cross_product(P a, P b) { return (conj(a)*b).imag(); }

/** Problem0021 : Parallelism **/
int main()
{
	int N; cin>>N;
	rep(i, N) {
		P a, b;
		D x1, y1, x2, y2;
		cin>>x1>>y1>>x2>>y2;
		a.X = (x2 - x1);
		a.Y = (y2 - y1);
		
		cin>>x1>>y1>>x2>>y2;
		b.X = (x2 - x1);
		b.Y = (y2 - y1);
		
		if (sig(cross_product(a, b))==0) {
			cout << "YES" << endl;
		} else {
			cout << "NO" << endl;
		}
	}
}