Go: package main

import "fmt"

func abs(x float64) float64 {
	if x < 0 {
		return -x
	}
	return x
}

func main() {
	var n int
	var x1, y1, x2, y2, x3, y3, x4, y4 float64
	fmt.Scan(&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)
		if abs((x2-x1)*(y4-y3) - (y2-y1)*(x4-x3)) < 1e-10 {
			fmt.Println("YES")
		} else {
			fmt.Println("NO")
		}
	}
}

C++: 
#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cmath>
#include <string>
#include <cstring>
#include <vector>
#include <map>
#include <utility>
#include <complex>
#include <stack>

using namespace std;

double dot(double x1,double y1,double x2,double y2){return x1*y2 - x2*y1;}

int main(){
	int t;
	cin>>t;
	while(t--){
		double x[4],y[4];
		for (int i = 0; i < 4; i++)
		{
			cin>>x[i]>>y[i];
		}

		double ans = dot(x[1] - x[0],y[1]-y[0],x[3]-x[2],y[3]-y[2]);
		if(-1e-8 <= ans && ans <= 1e-8) puts("YES");
		else puts("NO");
	}

	return 0;
}

C++:  
 #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 #include<stack>
 
 using namespace std;
 
 double abs(double x)
 {
   if(x<0) return -x;
   else return x;       
 }
  
 int main()
 {
   double x1,y1,x2,y2,x3,y3,x4,y4;
   int n;
   cin>>n;
   while(n--)
   {
     cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
    
       double k1=(y2-y1)/(x2-x1);
       double k2=(y4-y3)/(x4-x3);
       double judge=(x2-x1)*(y4-y3)-(y2-y1)*(x4-x3);
       if((abs(judge)<0.000000000001) cout<<"YES"<<endl;
       else cout<<"NO"<<endl;     
           
   }
   //while(1);      
   return 0;    
 }

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

class Point
{
public:
	double m_X;
	double m_Y;
	Point(double x, double y)
		: m_X( x )
		, m_Y( y )
	{}
};

class LineSegment
{
private:
	Point m_Start;
	Point m_End;
public:
	LineSegment(Point start, Point end)
		: m_Start( start )
		, m_End( end )
	{}
	double CalcSlope()
	{
		if ( m_End.m_Y == m_Start.m_Y)
		{
			return 0.0;
		}

		if (m_End.m_X == m_Start.m_X)
		{
			return 9999999999.0;
		}
	
		return((m_End.m_Y - m_Start.m_Y) / (m_End.m_X - m_Start.m_X));
	}
};

int main()
{
	int n = 0;
	double x1 = 0;
	double y1 = 0;
	double x2 = 0;
	double y2 = 0;
	double x3 = 0;
	double y3 = 0;
	double x4 = 0;
	double y4 = 0;

	std::cin >> n;

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

		Point start1{ x1, y1 };
		Point end1{ x2, y2 };
		Point start2{ x3, y3 };
		Point end2{ x4, y4 };

		LineSegment line1{ start1, end1 };
		double line1_slope = line1.CalcSlope();

		LineSegment line2{ start2, end2 };
		double line2_slope = line2.CalcSlope();

		if (line1_slope == line2_slope)
		{
			std::cout << "YES" << std::endl;
		}
		else
		{
			std::cout << "NO" << std::endl;
		}
	}

	return 0;
}

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

using namespace std;

int main(void){

	double x1,y1,x2,y2,x3,y3,x4,y4;
	int n;

	cin>>n;

	for(int i=0;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++: #include <iostream>
using namespace std;
int main(void)
{
    double x[4];
    double y[4];
    int n;
    cin >> n;
    while(n--){
        for(int i=0; i<4; i++){
            cin >> x[i] >> y[i];
        }
        double d1,d2;
        d1 = (y[1]-y[0])/(x[1]-x[0]);
        d2 = (y[3]-y[2])/(x[3]-x[2]);
        if(d1==d2) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return 0;
}

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

using namespace std;

typedef complex<double> comp;

const double EPS = 1e-10;

int main(){
  int n;
  cin >> n;
  while(n--){
    comp p[4];
    for(int i=0; i<4; i++){
      double x,y;
      cin >> x >> y;
      p[i] = comp(x,y);
      p[i] -= p[i/2*2]*(double)(i%2);
    }
    if(abs(sin(arg(p[1]/p[3]))) < EPS)
      cout << "YES";
    else cout << "NO";
    cout << endl;
  }
  return 0;
}

C++: #include<cstdio>
#include<iostream>
#include<cmath>
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;
        if(((x1-x2)*(y1-y2))-((x3-x4)*(y3-y4))==0||((x1-x2)*(y1-y2))-((x3-x4)*(y3-y4))==-0)cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
int main(void) {
    int n;
    double x1,y1,x2,y2,x3,y3,x4,y4;
    double EPS = 1e-10;
    cin>>n;
    for (int i=0;i<n;i++) {
        cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
        if ( abs((y4-y3)*(x2-x1)-(y2-y1)*(x4-x3))<EPS ) {
            cout<<"YES"<<endl;
        } else {
            cout<<"NO"<<endl;
        }
    }
}

C++: #include<iostream>
using namespace std;
int main(){
	double x1, y1, x2, y2, x3, y3, x4, y4, p1, p2;
	int n;
	cin >> n;
	for(int i = 0; i < n; i++){
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		p1 = (y2 - y1) / (x2 - x1);
		p2 = (y4 - y3) / (x4 - x3);
		if(p1 == p2)
			cout << "YES" << endl;
		else
			cout << "NO" << endl;	
	}
}

C++: #include <iostream>
#include <complex>
 
using namespace std;
 
typedef complex<double> P;
 
#define EPS (1e-10)
#define EQ(a, b) (abs((a) - (b)) < EPS)
#define EQV(a, b) ((EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag()))
 
double cross(P a, P b) 
{
    return (a.real() * b.imag() - a.imag() * b.real());
}
 
bool is_parallel(P a1, P a2, P b1, P b2) 
{
    return EQ(cross(a1 - a2, b1 - b2), 0.0);
}
 
 
int main()
{
    int num;
    double  x1, y1, x2, y2, x3, y3, x4, y4;
     
    cin >> num;
    for (int i = 0; i < num; i++) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
         
        if (is_parallel(P(x1, y1), P(x2, y2), P(x3, y3), P(x4, y4)))
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }
 
    return 0;
}

C++: #include<stdio.h>
 
int main(void){
    double x[4],y[4];
    int n,i;
     
    scanf("%d",&n);
     
    for(i=0;i<n;i++){
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x[0],&y[0],&x[1],&y[1],&x[2],&y[2],&x[3],&y[3]);
         
        if(((y[1]-y[0])/(x[1]-x[0])) == ((y[3]-y[2])/(x[3]-x[2])))printf("YES\n");
        else printf("NO\n");
    }
     
    return 0;
}

C++: #include<stdio.h>
#include<math.h>

int main()
{
	double x1, x2, x3, x4, y1, y2, y3, y4;
	int n, i;
	scanf("%d", &n);
	for (i = 0; i < n; i++)
	{
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		if ((x2 - x1)*(y4 - y3) - (x4 - x3)*(y4 - y3) == 0)
		{
			printf("NO\n");
		}
		else
		{
			printf("YES\n");
		}
	}
	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <string>
#include <cmath>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <bitset>
#include <functional>
#include <sstream>
#include <utility>
#include <cstdlib>
#include <cctype>
#include <climits>
#include <fstream>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<pair<int, int> > vpii;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n) for(int i=0;i<(n);++i)
#define foreach(itr,c) for(__typeof(c.begin()) itr=c.begin(); itr!=c.end(); itr++)
#define dbg(x) cout << #x"="<< (x) << endl
#define mp(a,b) make_pair((a),(b))
#define pb(a) push_back(a) 
#define in(x) cin >> x;
#define all(x) (x).begin(), (x).end()
#define INF 2147483600
#define fi first
#define se second
using namespace std;
const double eps = 1e-11;

int main(){
	int N;
	vd a(8);
	cin >> N;
	rep(i,N){
		rep(i,8){
			cin>>a[i];
		}
		if(abs((a[1]-a[3])*(a[4]-a[6])-(a[5]-a[7])*(a[0]-a[2]))< eps){
			cout<<"YES"<<endl;
		}else{
			cout<<"NO"<<endl;
		}
	}

}