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>
#include <algorithm>
#include <cmath>
using namespace std;
// typedef long long ll;
// const int maxn=10000000;
// bool is_prime[maxn];
// int solve1(int n){
// 	for(int i=0;i<=n;i++)is_prime[i]=true;
// 	int ans=0;
// 	for(int i=2;i<=n;i++){
// 		if(is_prime[i]){
// 			ans++;
// 			for(int j=i*2;j<=n;j+=i)is_prime[j]=false;
// 		}
// 	}
// 	return ans;
// }



int main(){
	int n;
	cin>>n;
	double x1,y1,x2,y2,x3,y3,x4,y4;
	while(n--){
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		if(x1==x2==x3==x4||y1==y2==y3==y4)cout<<"NO"<<endl;
		else {
			double t1=abs(y2-y1);
			double t2=abs(x2-x1);
			double t3=abs(y4-y3);
			double t4=abs(x4-x3);
			if(t2==0&&t4==0)cout<<"YES"<<endl;
			else if(t2*t4==0)cout<<"NO"<<endl;
			else if(t1/t2==t3/t4){
				double t=-t1/t2*x1+y1;
				if(y3==t1/t2*x3+t)cout<<"NO"<<endl;
				else cout<<"YES"<<endl;
			}
			else cout<<"NO"<<endl;
		}
	}
	return 0;
}

C++: #include<iostream>
#include<complex>
#include<cmath>
using namespace std;
typedef complex<double> xy_t;

xy_t P[4];

double cross(xy_t p,xy_t q){
    return (conj(p)*q).imag();
}

int main(){
    int n;
    double eps=pow(10,-11);
    cin>>n;
    for(int i=0;i<n;++i){
        for(int j=0;j<4;++j){
            double x,y;
            cin>>x>>y;
            P[j]=xy_t(x,y);
        }
        
        xy_t a[2]={
            P[0]-P[1],
            P[2]-P[3]
        };

        bool flag=(abs(cross(a[0],a[1]))<eps);

        cout<<(flag?"YES":"NO")<<"\n";

    }
}

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

int main(){
	double x1 , y1 , x2 , y2 , x3 , y3 , x4 , y4;
	double a , b;
	int n=0;
	cin >> n;
	for(int i=0 ; i<n ; ++i){
		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 <algorithm>
#include <cmath>
#include <climits>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <cassert>
#include <functional>

using namespace std;

#define LOG(...) printf(__VA_ARGS__)
//#define LOG(...)
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define CLR(a) memset((a), 0 ,sizeof(a))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int dx[] = { -1, 0, 1, 0 }; const int dy[] = { 0, 1, 0, -1 };

int main() {
	int n;
	cin>>n;
	while (n--){
		vector<pair<double, double>> pos(4);
		REP(i, 4){
			cin >> pos[i].first >> pos[i].second;
		}

		if ((pos[1].first - pos[0].first)*(pos[3].second - pos[2].second) - (pos[1].second - pos[0].second)*(pos[3].first - pos[2].first)==0.0)
			cout << "YES"<<endl;
		else
			cout << "NO"<<endl;
		
	}
}

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

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

  for(int i=0;i<n;i++){
    complex<double> v[4];
    for(int j=0;j<4;j++){
      double x,y;
      cin >> x >> y;
      v[j]=complex<double>(x,y);
    }

    if(abs((conj(v[1]-v[0])*(v[3]-v[2])).imag())<0.000001)
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }

  return 0;
}

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

int main(){
	int n;
	double a1,a2,b1,b2;
	double c1,c2,d1,d2;
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>a1>>a2>>b1>>b2>>c1>>c2>>d1>>d2;
		if( (b2-a2)/(b1-a1) == (d2-c2)/(d1-c1) ){
			cout<<"YES"<<endl;
		}else{ 
			cout<<"NO"<<endl;
		}

	}

}

C++: #include<iostream>

int main(){
  int n;
  double x1,y1,x2,y2,x3,y3,x4,y4;
  const double e = 1e-10;

  std::cin >> n;

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

    if((x1*y3+x2*y4+y2*x3+y1*x4)-(x2*y3+x1*y4+y1*x3+y2*x4)<e){
      std::cout << "YES" << std::endl;
    }else{
      std::cout << "NO" << std::endl;
    }
  }
}

C++: #include<iostream>
#include<cmath>
using namespace std;
int main(){
  int n;
  cin >> n;
  while(n--){
    double x[4],y[4];
    for(int i=0;i<4;i++) cin >> x[i] >> y[i]; 
    if(abs((x[3]-x[2])*(y[1]-y[0])-(x[1]-x[0])*(y[3]-y[2])) < 1e-10){
      cout<<"YES"<<endl;
    else
      cout<<"NO"<<endl;
  }
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <sstream>
#include <utility>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;
#define all(c) (c).begin(), (c).end()
#define loop(i,a,b) for(ll i=a; i<ll(b); i++)
#define rep(i,b) loop(i,0,b)
#define each(e,c) for(auto&e:c)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define lb lower_bound
#define ub upper_bound
#ifdef DEBUG
#define dump(...) (cerr<<#__VA_ARGS__<<" = "<<(DUMP(),__VA_ARGS__).str()<<" ["<<__LINE__<<"]"<<endl)
struct DUMP:ostringstream{template<class T>DUMP &operator,(const T&t){if(this->tellp())*this<<", ";*this<<t;return *this;}};
#else
#define dump(...)
#endif
 
int main(){
    int n; cin>>n;
    rep(_,n){
        double x[2][2], y[2][2];
        rep(i,2){
            rep(j,2){
                double x_,y_;
                cin >> x_ >> y_;
                x[i][j] = x_;
                y[i][j] = y_;
            }
        }
        double dx1=x[0][0]-x[0][1], dy1=y[0][0]-y[0][1];
        double dx2=x[1][0]-x[1][1], dy2=y[1][0]-y[1][1];
        dump(dx1,dx2,dy1,dy2);
        puts(dx1*dy2==dx2*dy1 ? "YES" : "NO");
    }
}

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

double abs_(double a)
{
	if (a > 0) return a;
	else return -a;
}

int main()
{
	int n;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		double x[4], y[4];
		double a, b;
		cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> x[3] >> y[3];
		a = (x[2] - x[3]) * (y[0] - y[1]);
		b = (x[0] - x[1]) * (y[2] - y[3]);
		bool h = abs_(a - b) < 0.0000000001;
		if (h)
		{
			cout << "YES" << endl;
		}
		else
			cout << "NO" << endl;
	}


	return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
int main(){
  double a[2],b[2],c[2],d[2];
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    for(int j=0;j<2;j++) cin >> a[j] >> b[j] >> c[j] >> d[j];
    if(a[0]-b[0]==0&&c[0]-d[0]==0) cout << "YES" << endl;
    else if(a[1]-b[1]==0&&c[1]-d[0]==0) cout << "YES" << endl;
    else if(fabs(a[0]*d[0]+a[1]*b[1])==fabs(b[0]*d[0]+b[1]*d[1])) cout << "YES" << endl;
    else cout << "NO" << endl;
  }   
  return 0;
}