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;

// 許容する誤差ε
#define EPS (1e-10)
// 2つのスカラーが等しいかどうか
#define EQ(a,b) (abs((a)-(b)) < EPS)

// 外積 (cross product) : a×b = |a||b|sinΘ
double cross(P a, P b) {
  return (a.real() * b.imag() - a.imag() * b.real());
}

// 2直線の平行判定 : 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()
{
	double x1[100], y1[100], x2[100], y2[100], x3[100], y3[100], x4[100], y4[100]; int n;

	cin >> n;

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

	for (int i = 0; i < n; i++)
	{
		if ((x2[i] - x1[i]) / (y2[i] - y1[i]) == (x4[i] - x3[i]) / (y4[i] - y3[i]))
		{
			cout << "YES" << endl;
		}
		else
		{
			cout << "NO" << endl;
		}
	}

	return 0;
}

C++: #include <iostream>
#include <complex>
#include <cmath>
using namespace std;
typedef complex<double> xy_t;
double cross_product(xy_t a, xy_t b){ return (conj(a)*b).imag();}
const double eps = 1e-11;
double x[4],y[4];
int N;
int main() {
	cin >> N;
	for(int t=0;t<N; ++t){
		for(int i=0; i<4; ++i){
			cin >> x[i] >>y[i];
		}
		xy_t a[2] ={
			xy_t(x[0],y[0])-xy_t(x[1],y[1]),
			xy_t(x[2],y[2])-xy_t(x[3],y[3])
		};
		bool p = abs(cross_product(a[0], a[1])) < eps;
		cout << (p ? "YES" : "NO" ) << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <string.h>
#include <string>
#include <algorithm>
#include <vector>
#define _USE_MATH_DEFINES
#include <math.h>

#define abs(a) (a > 0 ? a:-a)
using namespace std;

void q21() {
	int N;
	cin >> N;
	for (int i = 0; i < N; i++) {
		double ax, ay, bx, by, cx, cy, dx, dy;
		cin >> ax >> ay >> bx >> by >> cx >> cy >> dx >> dy;
		if (bx - ax == 0 || dx - cx == 0) {
			if (bx - ax == dx - cx) {
				cout << "YES" << endl;
				continue;
			}
		}
		else if (abs((by - ay) / (bx - ax) - (dy - cy) / (dx - cx)) <= 0.00001 ){
			cout << "YES" << endl;
			continue;
		}
		cout << "NO" << endl;
	}
}

int main() {
	q21();
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <stack>
#include <cstdlib>
#include <queue>
#include <vector>
#include <set>
#include <stdio.h>
#include <cstdio>
#include <cmath>
#include <complex> 
using namespace std;
typedef complex<double> xy_t;
xy_t P[2];
int N;
double x,y,z,w;
const double eps = 1e-11;
int main() {
cin >> N;
for( int j = 0;j < N ;j++) {
for(int i = 0;i < 2;i++) {
	cin >> x >> y >> z >> w;
	P[i] = xy_t(z-x,w-y);
}
	double s =  (P[0].real()*P[1].imag()-P[0].imag()*P[1].real())/2;
bool p;
if (abs(s) < eps) p = true;
else p = false;
cout << (p ? "YES" : "NO") << endl;
}
}

C++: #include <algorithm>
#include <vector>
#include <complex>
using namespace std;
//---------------------------------------------------------------
using Point = complex<double>;
const double eps = 1e-10;
double cross(const Point &a, const Point &b) { return imag(conj(a) * b); }
//---------------------------------------------------------------

#include <iostream>
int main() {
    int n; cin >> n;
    while (n--) {
        vector<double> x(4), y(4);
        for (int i = 0; i < 4; i++) {
            cin >> x[i] >> y[i];
        }
        Point v[2] = {
            Point(x[0], y[0]) - Point(x[1], y[1]),
            Point(x[2], y[2]) - Point(x[3], y[3])
        };
        double res = cross(v[0], v[1]);
        cout << (abs(res) < eps ? "YES" : "NO") << endl;
    }
    return 0;
}

C++: #include <iostream>

using namespace std;

class Vector2D {
  private:
    double x;
    double y;
  public:
    Vector2D(double, double);
    Vector2D operator - (Vector2D vec2) {
      double newX = this->x - vec2.x;
      double newY = this->y - vec2.y;
      Vector2D newvec(newX, newY);
      return newvec;
    }
    double GetX() { return x; };
    double GetY() { return y; };
    bool isParallel(Vector2D);
};

Vector2D::Vector2D(double _x, double _y) {
  x = _x;
  y = _y;
}

bool Vector2D::isParallel(Vector2D vec2) {
  if (x == 0 && vec2.GetX() == 0)
    return true;
  else if (y == 0 && vec2.GetY() == 0)
    return true;
  else if (vec2.GetX()/x == vec2.GetY()/y)
    return true;
  else
    return false;
}

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

  double x1,y1,x2,y2,x3,y3,x4,y4;
  for (int dataset_i = 0; dataset_i < n; dataset_i++) {
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    Vector2D vecAB = Vector2D(x2,y2) - Vector2D(x1,y1);
    Vector2D vecCD = Vector2D(x4,y4) - Vector2D(x3,y3);
    cout << (vecAB.isParallel(vecCD) ? "YES" : "NO") << endl;
  }

  return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
  int n;
  cin >> n;
  double ax,ay,bx,by,cx,cy,dx,dy,ans1,ans2,x,y;
   for(int i=0;i<n;i++){
     cin >> ax >> ay >> bx >> by >> cx >> cy >> dx >> dy;
     x = ax - bx;
     y = ay - by;
     ans1 = y / x;
     x = cx - dx;
     y = cy - dy;
     ans2 = y / x;
     if(ans1==ans2) cout << "YES" << endl;
     if(ans1!=ans2) cout << "NO" << endl;
   }
   return 0;
}

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

class vec2d {
public:
  double x,y;
  void set(double x=0.0,double y=0.0) {
    this->x = x;
    this->y = y;
  }
};
double cross(vec2d A,vec2d B) {
  return (A.x*B.y - B.x*A.y);
}
bool is_paralles(vec2d A,vec2d B) {
  if(cross(A,B) == 0.0) return true;
  return false;
}

int main() {
  int n;
  vec2d AB,CD;
  double 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;
    AB.set(x2-x1,y2-y1);
    CD.set(x4-x3,y4-y3);
    if(is_paralles(AB,CD)) {
      cout << "YES" << endl;
    }
    else {
      cout << "NO" << endl;
    }
  }
}

C++: #include<iostream>

using namespace std;

int main(){
	int n;
	cin >> n;
	while (n--){
		double a[2], b[2], c[2], d[2];
		cin >> a[0] >> a[1] >> b[0] >> b[1] >> c[0] >> c[1] >> d[0] >> d[1];
		if ((b[0] - a[0])*(d[1] - c[1]) - (b[1] - a[1])*(d[0] - c[0]) == 0) cout << "YES" << endl;
		else cout << "NO" << 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((y1-y2)/(x1-x2)==(y3-y4)/(x3-x4))cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}