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++: // 2014/07/21 Tazoe

#include <iostream>
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;

//		int X1 = (int)(x1*100000.0);
//		int Y1 = (int)(y1*100000.0);
//		int X2 = (int)(x2*100000.0);
//		int Y2 = (int)(y2*100000.0);
//		int X3 = (int)(x3*100000.0);
//		int Y3 = (int)(y3*100000.0);
//		int X4 = (int)(x4*100000.0);
//		int Y4 = (int)(y4*100000.0);

//		long long ABX = X2-X1;
//		long long ABY = Y2-Y1;
//		long long CDX = X4-X3;
//		long long CDY = Y4-Y3;

		double abx = x2-x1;
		double aby = y2-y1;
		double cdx = x4-x3;
		double cdy = y4-y3;

		if(abx*cdy==aby*cdx){
			cout << "YES" << endl;
		}
		else{
			cout << "NO" << endl;
		}
	}

	return 0;
}

C++: #include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define int long long
using namespace std;
typedef pair<int, int> P;

const double EPS = 1e-10;

signed main(){
    int n;
    cin >> n;
    double x[4], y[4];
    rep(i, 0, n){
        rep(j, 0, 4){
            cin >> x[j] >> y[j];
        }
        double h1, h2;
        int cnt = 0;
        if(fabs(x[1] - x[0] < EPS)) cnt++;
        else h1 = (y[1] - y[0]) / (x[1] - x[0]);
        if(fabs(x[3] - x[2]) < EPS) cnt++;
        else h2 = (y[3] - y[2]) / (x[3] - x[2]);
        if(fabs(h1 - h2) < EPS || cnt == 2) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
}

C++: #include<iostream>
const double eps = 1e-11;
using namespace std;

int main() {
	double x[8], y[8], z;
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int k = 0; k < 4; k++) {
			cin >> x[k] >> y[k];
		}
		x[0] -= x[1];
		y[0] -= y[1];
		x[2] -= x[3];
		y[2] -= y[3];
		z = x[0] * y[2] - x[2] * y[0];
		if (-eps < z &&z < eps) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<math.h>
#include<vector>
using namespace std;
#define EPS (1e-10)
#define equals(a,b) (fabs((a) - (b)) < EPS)
class Point{
public:
  double x,y;
  Point(double x = 0, double y = 0):x(x), y(y) {}
  Point operator + (Point p){
    return Point(x+p.x,y+p.y);
  }
  Point operator - (Point p){
    return Point(x-p.x,y-p.y);
  }
  Point operator * (double k){
    return Point(x*k,y*k);
  }
  Point operator / (double k){
    return Point(x/k,y/k);
  }
  bool operator == (const Point &p) const{
    return fabs(x-p.x) < EPS && fabs(y-p.y) < EPS;
  }
};
typedef Point Vector;
typedef vector<Point> Polygon;
double dot(Vector a,Vector b){
  return a.x * b.x + a.y * b.y;
}
double cross(Vector a,Vector b){
  return a.x * b.y - a.y * b.x;
}

bool isParallel(Vector a,Vector b){
  return equals(cross(a,b),0.0);
}
int main(){
  Point a,b,c,d;
  int n;
  cin >> n;
  int i;
  for(i=0;i<n;i++){
    cin >> a.x >> a.y >> b.x >> b.y >> c.x >> c.y >> d.x >> d.y;
    if(isParallel(a-b,c-d)) cout << "YES" <<endl;
    else cout << "NO" <<endl;
  }
  return 0;
}

C++: #include <iostream>
#include<cmath>
#include<algorithm>
#include<string>
#include<map>
#include<vector>
using namespace std;
int main (void) {
    int n;
    double x1,x2,x3,x4,y1,y2,y3,y4;
    cin >> n;
    while(n--){
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        if((x1-x2)*(y3-y4)==(x3-x4)*(y1-y2))    cout << "YES\n";
        else    cout << "NO\n";
    }
    return 0;
}

C++: // clang-format off
#include <bits/stdc++.h>
#define int long long
#define main signed main()
// #define main int main()
#define loop(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) loop(i, 0, n)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define prec(n) fixed << setprecision(n)
#define stlice(from, to) substr(from, (to) - (from) + 1)
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
using namespace std;
using pii = pair<int, int>;
using vi = vector<int>;
using vd = vector<double>;
using vc = vector<char>;
using vb = vector<bool>;
using vs = vector<string>;
using vpii = vector<pii>;
using vvi = vector<vi>;
using vvb = vector<vb>;
using vvpii = vector<vpii>;
template<typename A> using fn = function<A>;
constexpr int INF = sizeof(int) == sizeof(long long) ? 1000000000000000000LL : 1000000000;
constexpr int MOD = 1000000007;
constexpr double PI = acos(-1);
template<typename A, typename B> bool cmin(A &a, const B &b) { return a > b ? (a = b, true) : false; }
template<typename A, typename B> bool cmax(A &a, const B &b) { return a < b ? (a = b, true) : false; }
constexpr bool odd(const int &n) { return n & 1; }
constexpr bool even(const int &n) { return !odd(n); }
template<typename V> constexpr typename V::value_type sum(const V &v) { return accumulate(all(v), 0); }
void solve();
main { solve(); return 0; }
// clang-format on

void solve() {
  int n;
  cin >> n;
  rep(i, n) {
    double x1, y1, x2, y2, x3, y3, x4, y4;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if (x1 == x2) {
      if (x3 == x4) {
        cout << "YES" << endl;
      } else {
        cout << "NO" << endl;
      }
      continue;
    }
    if ((y1 - y2) / (x1 - x2) == (y3 - y4) / (x3 - x4)) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }
}

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

typedef complex<double> P;

// 許容する誤差ε
#define EPS (1e-10)
// 2つのスカラーが等しいかどうか
#define EQ(a,b) (abs((a)-(b)) < EPS)

// 外積 (cross product) : a×b = |a||b|sinΘ
double cross(P a, P b) {
  return (a.real() * b.imag() - a.imag() * b.real());
}

// 2直線の平行判定 : a//b <=> cross(a, b) = 0
int is_parallel(P a1, P a2, P b1, P b2) {
  return EQ( cross(a1-a2, b1-b2), 0.0 );
}

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

	cin >> n;
	for(int i=0 ; i<n ; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		P a(x1,y1), b(x2,y2), c(x3,y3), d(x4,y4);
		if( is_parallel(a,b,c,d) ){
			cout << "YES" << endl;
		}else{
			cout << "NO" << endl;
		}
	}
}

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

int main(){
	double 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((y1-y2)*(x3-x4) == (y3-y4)*(x1-x2)){
			cout << "YES" << endl;
		}else{
			cout << "NO" << endl;
		}
	}
	return 0;
}

C++: #define _USE_MATH_DEFINES
#include <iostream>
#include <string>
#include <utility>
#include <algorithm>
#include <queue>
#include <map>
#include <fstream>
#include <array>
#include <utility>
#include <functional>
#include <stack>
#include <math.h>

using namespace std;
typedef long long Int;
#define REP(i , x) for(Int i = 0 ; i < x ; ++i)
typedef pair<int, int> P;


int main()
{
	int n;
	cin >> n;

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

		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

		double dx1 = x2 - x1;
		double dx2 = x4 - x3;
		double dy1 = y2 - y1;
		double dy2 = y4 - y3;

		string ans = (dy1 / dx1 == dy2 / dx2) ? "YES" : "NO";

		if (!dx1 || !dx2){
			ans = "NO";
		}

		cout << ans << endl;
	}
	return 0;
}