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<string>
#include<iostream>
#include<algorithm>
using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		long double a[4][2];
		for (int I = 0; I< 4; I++)
		{
			for (int k = 0; k < 2; k++)
			{
				cin >> a[I][k];
			}
		}
		long double x, y;
		x = (a[1][0] - a[0][0]);
		y = (a[3][0] - a[2][0]);
		x = (x) ? (a[1][1] - a[0][1]) / x : 0;
		y = (y) ? (a[3][1] - a[2][1]) / y : 0;
		if (x == y)cout << "YES" << endl;
		else cout << "NO" << endl;
	}

}
/*
		x = (a[1][1] - a[0][1]) / (a[1][0] - a[0][0]);
		y = (a[3][1] - a[2][1]) / (a[3][0] - a[2][0]);*/

C++: #include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < (int)n; ++i)
#define repf(i, f, l) for(int i = f; i < (int)l; ++i)

#ifdef ONLINE_JUDGE
#define DEBUG false
#else
#define DEBUG true
#endif

#define pb emplace_back
#define lb lower_bound
#define ul unsigned long
#define ull unsigned long long
#define ll long long
#define INF 1000000007
#define MOD 1000000007
#define fs first
#define sd second

#define ALL(c) (c).begin(),(c).end()

#define DBG0(x)    {if(DEBUG){ cout << #x << ": " << x << "\t"; }}
#define DBG(x)     {if(DEBUG){DBG0(x); cout << endl;}}
#define DBG2(x, y) {if(DEBUG){DBG0(x); DBG(y);}}
#define DBG3(x, y, z) {if(DEBUG){DBG0(x); DBG2(y, z);}}
#define DBG4(w, x, y, z) {if(DEBUG){DBG0(w); DBG3(x, y, z);}}

const double EPS = 1.0e-14;
template <class T>
ostream& operator<<(ostream& os, vector<T> xs){ for(T x: xs) os << x << ' '; return os; }
template <class S, class T>
ostream& operator<<(ostream& os, pair<S,T> st){ os << "(" << st.first << "," << st.second <<")"; return os; }

typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<ul> vul;
typedef vector<ull> vull;
typedef vector<bool> vbl;
typedef pair<int, int> pii;

typedef complex<double> Point;
istream& operator>>(istream& is, Point &pt){
    double a, b;
    is >> a >> b;
    pt.real(a); pt.imag(b);
    return is;
}

ostream& operator<<(ostream& os, const Point &pt){
    os << make_pair(pt.real(), pt.imag());
    return os;
}

inline double cross(const Point &a, const Point &b){
    return (a.real()*b.imag() - a.imag()*b.real());
}

inline double myarg(Point &a){
    double ret = arg(a);
    return (ret >= 0 ? ret : ret + 2*M_PI);
}

inline double signed_area(const Point &a, const Point &b, const Point &c){
    Point ab = b - a;
    Point ac = c - a;
    return cross(ab, ac) / 2.0;
}

inline bool intersect(const Point &a1, const Point &a2, const Point &b1, const Point &b2) {
    return ((cross(a2-a1, b1-a1) * cross(a2-a1, b2-a1) < EPS) &&
            (cross(b2-b1, a1-b1) * cross(b2-b1, a2-b1) < EPS));
}

void sort_by_arg(vector<Point> &pts){
    sort(pts.begin(), pts.end(), [](Point a, Point b){ return (myarg(a) < myarg(b)); });
}

double area(vector<Point> &vs){
    int n = vs.size();
    vector<Point> pts(n - 1);
    rep(i, n - 1) pts[i] = vs[i + 1] - vs[0];
    sort_by_arg(pts);
    double ans = 0.0;
    rep(i, n - 1) ans += cross(vs[i], vs[i + 1]);
    return ans / 2.0;
}

int main(void){

    int n;
    cin >> n;
    while(n-->0){
        Point a,b,c,d;
        cin >> a >> b >> c >> d;
        if(abs(cross(b - a, d - c)) < EPS)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }
    return 0;
}

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

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

  cin >> n;

  for (int i = 1; i <= n; i++){
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    if (x2 - x1 / (y2 - y1) == x4 - x3 / (y4 - y3)){
      cout << "YES" << endl;
    }
    else {
      cout << "NO" << endl;
    }
  }

  return 0;
}

C++: //二次元幾何ライブラリ
#include <iostream>
#include <cstdio>
#include <complex>
#include <map>
#include <vector>

using namespace std;

#define rep(i, n) for (int i = 0; i < int(n); ++i)
#define MC make_pair
#define ML make_pair 
#define X real()
#define Y imag()
typedef double D;
typedef bool B;
typedef complex<D> P;   //点
typedef complex<D> V;   //ベクトル
typedef pair<P,P>  L;   //直線
typedef pair<P,D>  C;   //円
typedef vector<P>  Pol; //多角形

//変数名定義
const D INF = 1e100;
const D EPS = 1e-8;
const D PI = acos(-1);

template<class T> bool operator==(T a, T b){return abs(a - b )< EPS;}
template<class T> int sig(T r) {return (r==0||r==-0) ? 0 : r > 0 ? 1 : -1;}

P p3,p4;
int num;

// ベクトルaの絶対値を求める
// D Length = abs(a);

// 2点a,b間の距離を求める
// D Distance = abs(a-b); 

// 内積 (Inner Product) : a・b = |a||b|cosθ 1
D IP(V a, V b) {
	return (a.real() * b.real() + a.imag() * b.imag());
}

// 外積 (Exterior Product) : |a×b| = |a||b|sinθ 2
D EP(V a, V b) {
	return (a.real() * b.imag() - a.imag() * b.real());
}

// 点の進行方向について 3
int ccw(P a, P b, P c) {              //点aと点bが与えられた問いに
  b -= a; c -= a;
  if (EP(b, c) > 0)   return +1;      // counter clockwise
  if (EP(b, c) < 0)   return -1;      // clockwise
  if (IP(b, c) < 0)     return +2;    // c--a--b on line
  if (norm(b) < norm(c)) return -2;   // a--b--c on line
  return 0;                           // a--c--b on line  aとbの線分判定はこれ
}

// 2直線の垂直判定 : a⊥b <=> IP(a, b) = 0 4needs1
B IsVecular(L l1, L l2) {
	return IP(l1.first-l1.second, l2.first-l2.second)==0.0;
}

// 2直線の平行判定 : a//b <=> EP(a, b) = 0 5needs2
B IsParallel(L l1, L l2) {
	return EP(l1.first-l1.second, l2.first-l2.second) < EPS;
}


//example : v1 = V(5.0,-3.0) p1=P(0.0,4.0)
int main(void){
	int n;
	cin >> n;
	for(int i=0;i<n;i++){
		P points[4];
		for(int j=0;j<4;j++)
			cin >> points[j].X >> points[j].Y;
		L l1,l2;
		l1=ML(points[0],points[1]);
		l2=ML(points[2],points[3]);
		if(IsParallel(l1,l2))
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}

C++: #include <stdio.h>

struct point{
	double x;
	double y;
};

double t(point p1, point p2){
	return (p1.y - p2.y)/(p1.x - p2.x);
}

int main(void){
	struct point p[4];
	int n = 0;
	scanf("%d", &n);
	for(int i=0;i<n;++i){
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf",
			&(p[0].x), &(p[0].y),
			&(p[1].x), &(p[1].y),
			&(p[2].x), &(p[2].y),
			&(p[3].x), &(p[3].y));
		if(t(p[0], p[1]) == t(p[2], p[3])){
			puts("YES");
		}else{
			puts("NO");
		}
	}
	return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <sstream>
#include <stdio.h>
#include <vector>
#include <stack>
#include <queue>
#include <algorithm>
#include <math.h>
#include <utility>
#include <string>
#include <ctype.h>
#include <cstring>
#include <cstdio>
#include <sstream>
#include <functional>

using namespace std;

#define FOR(i,k,n) for(int i = (k); i < (n); i++)
#define REP(i,n) FOR(i,0,n)
#define INF 114514810
#define ll long long
//#define scanf scanf_s

int main()
{
	int n;
	cin >> n;
	REP(i, n)
	{
		double xa, ya, xb, yb, xc, yc, xd, yd;
		cin >> xa >> ya >> xb >> yb >> xc >> yc >> xd >> yd;
		double ab, cd;
		if (xa == xb) ab = INF;
		else ab = (ya - yb) / (xa - xb);
		if (xc == xd) cd = INF;
		else cd = (yc - yd) / (xc - xd);
		if (ab == cd) cout << "YES" << endl;
		else cout << "NO" << endl;
		
	}
	return 0;
}