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 <complex>
#include <iostream>
using namespace std;
 
typedef complex<double> P;
 
// ??¨?±?????????????????
#define EPS (1e-10)
// 2??????????????????????????????????????????
#define EQ(a,b) (abs((a)-(b)) < EPS)
 
// ?????? (cross product) : a??b = |a||b|sin??
double cross(P a, P b) {
  return (a.real() * b.imag() - a.imag() * b.real());
}
 
// 2???´?????????????????? : 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()
{
  int n;
  cin >> n;
  for (int i = 0; i < n; i++){
    float x1, x2, x3, x4, y1, y2, y3, y4, a, b;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    a = (y2 - y1) / (x2 - x1);
    b = (y4 - y3) / (x4 - x3);
    if (a == b) cout << "YES" << endl;
    else cout << "NO" << endl;
  }
}

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

C++: #include <iostream>
#include <cmath>

using namespace std;
#define eps 1e-9

void solve()
{
	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[1] - x[0]) < eps && abs(x[3] - x[2]) < eps)
		{
			if(abs(y[1] - y[0]) < eps || abs(y[3] - y[2]) < eps)
			{
				cout << "NO" << endl;
			}
			else
			{
				cout << "YES" << endl;
			}
		}
		else if(abs(x[1] - x[0]) < eps || abs(x[3] - x[2]) < eps)
		{
			cout << "NO" << endl;
		}
		else if(abs((y[1] - y[0]) / (x[1] - x[0]) - (y[3] - y[2]) / (x[3] - x[2])) < eps)
		{
			cout << "YES" << endl;
		}
		else
		{
			cout << "NO" << endl;
		}
	}
}

int main()
{
	solve();
	return(0);
}

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


int main(int argc, char *argv[]){
	double n,x1 ,y1 ,x2 ,y2 ,x3 ,y3 ,x4 ,y4;
	cin>>n;
	for (int i = 0; i < n; ++i){
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		double s1=(y2-y1)/(x2-x1), s2=(y4-y3)/(x4-x3);
		if(s1==s2) cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}

    return 0;
}

C++: #include <iostream>
#include <fstream>
#include <typeinfo>
#include <vector>
#include <cmath>
#include <set>
#include <map>
#include <string>
#include <algorithm>
#include <cstdio>
#include <queue>
#include <iomanip>
#include <cctype>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<char> vc;
typedef vector<vc> vvc;
typedef vector<bool> vb;
typedef vector<vb> vvb;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef pair<int,P> pip;
typedef vector<pip> vip;
const int inf=1<<25;
const double pi=acos(-1);
const double eps=1e-8;
const vi emp;

double add_double(double a,double b){
	if(abs(a+b)<eps*(abs(a)+abs(b))) return 0;
	return a+b;
}

struct point{
	double x,y;
	point operator+(point p){
		return point{add_double(x,p.x),add_double(y,p.y)};
	}
	point operator-(point p){
		return point{add_double(x,-p.x),add_double(y,-p.y)};
	}
	point operator*(double p){
		return point{x*p,y*p};
	}
	point operator/(double p){
		if(!p) return point{0,0}; 
		return point{x/p,y/p};
	}
	bool operator==(point p){
		return fabs(add_double(x,-p.x))<eps&&fabs(add_double(y,-p.y))<eps;
	}
	bool operator<(point p){
		if(fabs(x-p.x)>eps) return x<p.x;
		return y<p.y;
	}
};

typedef pair<point,point> pp;
typedef vector<point> VP;
const point O{0,0};

class Geom{
	public:
	double Length(point x,point y){
		point z=y-x;
		return sqrt(z.x*z.x+z.y*z.y);
	}
	double IP(point p,point q){
		return p.x*q.x+p.y*q.y;
	}
	double CP(point p,point q){
		return p.x*q.y-q.x*p.y;
	}
	string Counter_Clockwise(pp a,point x){
		point A=a.second-a.first;
		point X=x-a.first;
		double ip=IP(A,X),cp=CP(A,X),Al=Length(O,A),Xl=Length(O,X);
		if(cp>eps) return "Counter_Clockwise";
		if(cp<-eps) return "Clockwise";
		if(ip<-eps) return "Online_Back";
		if(Xl<Al||fabs(Xl-Al)<eps) return "On_Segment";
		return "Online_Front";
	}
	string Parallel_Orthogonal(pp a,pp b){
		point A=a.second-a.first,B=b.second-b.first;
		double ip=IP(A,B),cp=CP(A,B);
		string f=Counter_Clockwise(pp(a.first,a.second),b.first),s=Counter_Clockwise(pp(a.first,a.second),b.second);
		if(f[0]!='C'&&s[0]!='C') return "Agreement";
		if(cp<eps&&cp>-eps) return "Parallel";
		if(ip<eps&&ip>-eps) return "Orthogonal";
		else return "Commonly";
	}
	void Point_in(point& p){
		cin>>p.x>>p.y;
	}
};

int n;
VP p(4);
string s;

int main(){
	Geom geo;
	cin>>n;
	for(int i=0;i<n;i++){
		for(VP::iterator j=p.begin();j!=p.end();j++) geo.Point_in(*j);
		s=geo.Parallel_Orthogonal(pp(p[0],p[1]),pp(p[2],p[3]));
		if(s[0]=='P'||s[0]=='A') cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
}

C++: #include<iostream>
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++){
    cin >> a[0] >> a[1] >> b[0] >> b[1] >> c[0] >> c[1] >> d[0] >> d[1];
    if(a[0]-b[0]==0&&c[0]-d[0]==0) cout << "YES" << endl;
    else if(a[1]-b[1]==0&&c[1]-d[1]==0) cout << "YES" << endl;
    if((a[1]-b[1])/(a[0]-b[0])==(c[1]-d[1])/(c[0]-d[0])) cout << "YES" << endl;
    else cout << "NO" << endl;
  }   
  return 0;
}

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

using namespace std;

typedef complex<double> P;

#define EPS (1e-5)

double cross(P const& a, P const& b) {
  return imag(conj(a)*b);
}

bool is_parallel(P const& a1, P const& a2, P const& b1, P const& b2)
{ return abs(cross(a2-a1, b2-b1)) <= EPS; }

int main() {
  
  int N; cin >> N;
  for(int i=0; i<N; i++) {
    P ps[4];
    for(int i=0; i<4; i++)
      cin >> ps[i].real() >> ps[i].imag();
    printf("%s\n", (is_parallel(ps[0], ps[1], ps[2], ps[3]) ? "YES" : "NO" ));
  }
  
  return 0;
}

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