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<cstdio>
#include<iostream>
 
using namespace std;
 
int main(void){
 
    long 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(y2-y1==0){
            if(y4-y3==0) cout<<"YES"<<endl;
            else cout<<"NO"<<endl;
        }
        else if((x2-x1)/(y2-y1)==(x4-x3)/(y4-y3)) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
 
    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 EPS = 0.000000001;
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 (abs(x1 - x2) < EPS && abs(x3 - x4) < EPS) {
      cout << "YES" << endl;
      continue;
    }
    if (abs(x1 - x2) < EPS || abs(x3 - x4) < EPS) {
      cout << "NO" << endl;
      continue;
    }
    if (abs((y1 - y2) * (x3 - x4) - (y3 - y4) * (x1 - x2)) < EPS) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }
}

C++: #include <bits/stdc++.h>
#define rep(i,l,n) for(int i=l;i<n;i++)
using namespace std;

int main(){
	int n;
	cin>>n;
	rep(i,0,n){
		double x1,y1,x2,y2,x3,y3,x4,y4,m1,m2;
		cin>>x1>>y1>>x2>>y2>>x3>>y3>>x4>>y4;
		m1=(y2-y1)/(x2-x1);
		m2=(y4-y3)/(x4-x3);
		if(abs(m1-m2)<1e-13) cout<<"YES"<<endl;
		else cout<<"NO"<<endl;
	}
}

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

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

int main()
{
	const double EPS = 1e-10;
	const double PI = 4.*atan(1.);

	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;
		complex<double> A(x1,y1),B(x2,y2),C(x3,y3),D(x4,y4);
		double a = arg((A-B)/(C-D));
		cout << ( abs(a)<=EPS || abs(a)>=PI-EPS ? "YES" : "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>

const double eps = 1e-10;

int n;
//math.hライブラリでy1, y2...とかって変数名が使われてるらしい
double ix1, ix2, ix3, ix4;
double iy1, iy2, iy3, iy4;

//増加率を見る方法
int solve1() {
    double r1 = (iy2-iy1)/(ix2-ix1);
    double r2 = (iy4-iy3)/(ix4-ix3);
    return (fabs(r1-r2) < eps)
}

int main() {
    scanf("%d", &n);
    for(int i=0; i<n; ++i) {
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &ix1, &iy1, &ix2, &iy2, &ix3, &iy3, &ix4, &iy4);

        if(solve1()) {
            printf("YES\n");
        } else {
            printf("NO\n");
        }
    }
}

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

C++: #include <iostream>
using namespace std;
int main()
{
	int n, i, j;
	double x[4], y[4];

	cin >> n;

	for (i = 0; i < n; i++)
	{
		for (j = 0; j < 4; j++)
			cin >> x[j] >> y[j];

		if (((y[0]-y[1])/(x[0]-x[1])) == ((y[2]-y[3])/(x[2]-x[3])))
			cout << "YES" << endl;
		else
			cout << "NO" << endl;
	}
	return 0;
}

C++: #define _USE_MATH_DEFINES
#include <iostream>
#include <sstream>
#include <cmath>
#include <algorithm>
#include <queue>
#include <stack>

typedef long long ll;
using namespace std;

int main()
{
	int T;
	scanf("%d",&T);

	for(int t=0;t<T;t++){
		double pos[8];
		double eps = 10e-9;
		for(int i=0;i<8;i++){
			scanf("%lf",pos+i);
		}

		if((pos[0] == pos[2] && pos[0+4] == pos[2+4])
			|| !((pos[0] == pos[2] && pos[1] == pos[3]) || (pos[0+4] == pos[2+4] && pos[1+4] == pos[3+4]))
			&& abs((pos[1]-pos[3])/(pos[0]-pos[2])-(pos[1+4]-pos[3+4])/(pos[0+4]-pos[2+4]))<eps)
		{
			cout << "YES" << endl;
		}else
		{
			cout << "NO" << endl;	
		}
	}
}

C++: #include <iostream>
using namespace std;
int n;
double xa,xb,xc,xd,ya,yb,yc,yd,ta,tb,tc,td;
int main(){
	int i;
	while(cin >> n){
		for(i=0;i<n;i++){
			cin >> xa >> ya >> xb >> yb >> xc >> yc >> xd >> yd;
			ta = xb - xa;
			tb = yb - ya;
			tc = xd - xc;
			td = yd - yc;
			if(tb == td && ta == tc) cout << "YES" << endl;
			else cout << "NO" << endl;
		}
	}
}

C++: #include<iostream>
#include<complex>
using namespace std;
#define dff 1e-9
int main(){
  int n;
  cin >>n;
  while(n--){
    double x;
    complex<double> a[4];
    for(int i=0; i<4; i++) cin >>a[i].real()>>a[i].imag();
    for(int i=0; i<2; i++) a[i*2]-=a[i*2+1];
    x = a[0].real()*a[2].imag()-a[0].imag()*a[2].real();
    if(-dff<x && x<dff) cout <<"YES"<<endl;
    else cout <<"NO"<<endl;
  }
  return 0;
}