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 <string>
#include <vector>
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<stack>
#include<queue>
#include<cmath>
#include<algorithm>
#include<functional>
#include<list>
#include<deque>
#include<bitset>
#include<set>
#include<map>
#include<cstring>
#include<sstream>
#include<complex>
#include<iomanip>
#include<numeric>
#define X first
#define Y second
#define pb push_back
#define rep(X,Y) for (int (X) = 0;(X) < (Y);++(X))
#define rrep(X,Y) for (int (X) = (Y-1);(X) >=0;--(X))
#define repe(X,Y) for ((X) = 0;(X) < (Y);++(X))
#define peat(X,Y) for (;(X) < (Y);++(X))
#define all(X) (X).begin(),(X).end()
#define rall(X) (X).rbegin(),(X).rend()

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"{"; rep(i,t.size()) {os<<t[i]<<",";} os<<"}"<<endl; return os;}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}

int main(){
  ios_base::sync_with_stdio(false);
  cout<<fixed<<setprecision(0);
  int i,j,k,T;
  cin>>T;
  while(T--){
    double xx,yy,EPS=1e-6;
    ll x[4],y[4];
    rep(i,4){
      cin>>xx>>yy;
      x[i]=xx*1e5+EPS;
      y[i]=yy*1e5+EPS;
      //   cout<<x[i]<<","<<y[i]<<endl;
    }cout<<1e5<<endl;
    rep(i,2){
      x[i*2+1]-=x[i*2];
      y[i*2+1]-=y[i*2];
    }
    cout<<(x[1]*y[3]==x[3]*y[1]?"YES":"NO")<<endl;
  }
  return 0;
}

C++: /* AOJ 0021 Parallelism */

#include <iostream>
#include <cmath>

using namespace std;

const double EPS = 1e-9;

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

    cin >> n;

    while (n--) {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;

        if (fabs((x2 - x1) * (y4 - y3) - (x4 - x3) * (y2 - y1)) < EPS)
            cout << "YES" << endl;
        else
            cout << "NO" << endl;
    }

    return 0;
}

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

using namespace std;

int main(void) {
	int ds;
	cin >> ds;
	
	for(int i = 0; i < ds; i++) {
		double X[4], Y[4];
		double a1, a2;
		double b1, b2;
		
		for(int j = 0; j < 4; j++) {
			cin >> X[j] >> Y[j];
		}
		a1 = abs(Y[1] - Y[0]);
		b1 = (X[1] - X[0]);
		
		a2 = abs(Y[3] - Y[2]);
		b2 = (X[3] - X[2]);
		
		a1 /= b1;
		a2 /= b2;
		
		if(a1 == a2) {
			cout << "YES" << endl;
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

C++: #define scanf_s scanf
//#define gets_s gets
#include <stdio.h>
#include <string>
#include <iostream>
#include <math.h>
using namespace std;
#define MAX 201
#define _MAX 100
#define ABC 26
int main(void)
{
	int n, ans[_MAX] = { 0 };
	double x1, y1, x2, y2, x3, y3, x4, y4, a1, b1, a2, b2;
	scanf_s("%d", &n);
	for (int i = 0; i < n; ++i) {
		scanf_s("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);
		a1 = (y1 - y2) / (x1 - x2);
		b1 = y1 - (x1 * a1);
		a2 = (y3 - y4) / (x3 - x4);
		b2 = y3 - (x3 * a2);
		if (a1 == a2 && b1 != b2)
			++ans[i];
	}
	for (int i = 0; i < n; ++i) {
		if (ans[i])
			printf("YES");
		else
			printf("NO");
	}
}

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;

int main(){
	int N;
	vd a(8);
	cin >> N;
	rep(i,N){
		rep(i,8){
			cin>>a[i];
		}
		if(((a[1]-a[3])*(a[4]-a[6])-(a[5]-a[7])*(a[0]-a[2]))*((a[1]-a[3])*(a[4]-a[6])-(a[5]-a[7])*(a[0]-a[2]))<0.000001){
			cout<<"YES"<<endl;
		}else{
			cout<<"NO"<<endl;
		}
	}

}

C++: #include <cstdio>

using namespace std;

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

int main()
{
    int n; scanf( "%d\n", &n );
    for( int i = 0; i < n; i++ )
    {
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf %lf\n", &x1, &y1, &y2, &y2, &y3, &y3, &y4, &y4 );
        double dx1 = x2 - x1, dx2 = x4 - x3, dy1 = y2 - y1, dy2 = y4 - y3;
        if( (dx1 == 0 && dx2 == 0) || (dy1 == 0 && dy2 == 0 ) )
        {
            printf( "YES\n" );
        }
        else
        {
            dy1 /= dx1;
            dy2 /= dx2;
            if( dy1 == dy2 || dy1 == -dy2 )
            {
                printf( "YES\n" );
            }
        }
        printf( "NO\n" );
        n--;
    }
    return 0;
}

C++: #include <iostream>
using namespace std;
 
int main() {
    int N;
    double x1,y1,x2,y2,x3,y3,x4,y4;
    cin >> N;
    for (int n = 0; n != N; ++n) {
        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 <stdio.h>
#include <iostream>

using namespace std;

int main() {
	int k;
	double x1,x2,x3,x4,y1,y2,y3,y4;
	double katamuki1;
	double katamuki2;
	cin >> k;

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

		katamuki1 = (y2 - y1) / (x2 - x1);
		katamuki2 = (y4 - y3) / (x4 - x3);

		if (katamuki1 == katamuki2) {
			if(((y4 - y1) / (x4 - x1)) == ((y3 - y2) / (x3 - x2))) {
				cout << "NO" << endl;
			} else {
				cout << "YES" << endl;
			}
		} else {
			cout << "NO" << endl;
		}
	}
	return 0;
}

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

int main() {
  int n;
  cin >> n;
  while (n--) {
    float x[4], y[4];
    for (int i = 0; i < 4; i++) {
      cin >> x[i] >> y[i];
    }
    // if((x2-x1)*(y4-y3)==(x4-x3)*(y2-y1))cout<<"YES"<<endl;
    if ((x[1] - x[0]) * (y[3] - y[2]) == (x[3] - x[2]) * (y[1] - y[0])) {
      cout << "YES" << endl;
    } else {
      cout << "NO" << endl;
    }
  }
}