Go: package main

import (
        "fmt"
)

func main() {
        var x1, y1, x2, y2, x3, y3, px, py float64
        var oax, oay, obx, oby, opx, opy float64
        var det float64
        var s, t float64

        for {
                _, err := fmt.Scanf("%f %f %f %f %f %f %f %f", &x1, &y1, &x2, &y2, &x3, &y3, &px, &py)
                if err != nil {
                        break
                }
                oax = x2 - x1
                oay = y2 - y1
                obx = x3 - x1
                oby = y3 - y1
                opx = px - x1
                opy = py - y1
                det = oax*oby - obx*oay

                if det == 0 {
                        fmt.Printf("NO\n")
                } else {
                        s = (oby*opx - obx*opy) / det
                        t = (oax*opy - oay*opx) / det

                        if (s+t < 1 && s > 0 && t > 0) {
                                fmt.Printf("YES\n")
                        } else {
                                fmt.Printf("NO\n")
                        }
                }
        }
}

C++: #include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cstring>
#include <climits>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <vector>
#include <queue>
#include <string>
#include <complex>
using namespace std;

#define rep(i,n) for(int i=0; i<n; i++)
#define repa(i,s,e) for(int i=s; i<=e; i++)
#define repd(i,s,e) for(int i=s; i>=e; i--)

typedef long long ll;
typedef vector<int> vi;

// geometry library start

const double EPS = 1.0e-10;

class P {
public:
	double x, y;
	P() {}
	P(int x_, int y_) : x(x_), y(y_) {}
	P(double x_, double y_) : x(x_), y(y_) {}
	P(const P& p) : x(p.x), y(p.y) {}
	P operator+(const P& p) const { return P(x+p.x, y+p.y); } 
	P operator-(const P& p) const { return P(x-p.x, y-p.y); }
	P operator-() const { return P(-x, -y); }
	P operator*(double s) const { return P(x*s, y*s); }
	P operator/(double s) const { return P(x/s, y/s); }
	P& operator=(const P& p) { x=p.x; y=p.y; return (*this); }
	double dot(const P& p) const { return x*p.x + y*p.y; }
	double det(const P& p) const { return x*p.y - y*p.x; }
	double norm() const { return sqrt(x*x + y*y); }
	double norm2() const { return x*x + y*y; }
	P proj(const P& p) const { double k=det(p)/norm2(); return P(x*k, y*k); }
};

double tri(P a, P b, P c) { return (b - a).det(c - a); }
int sign(double r) { return r < -EPS ? -1 : r > EPS ? 1 : 0; }

class L {
public:
	P s, t;
	L() {}
	L(P s_, P t_) : s(s_), t(t_) {}
	L(const L& l) : s(l.s), t(l.t) {}
	L& operator=(const L& l) { s=l.s; t=l.t; return (*this); }
	double length() { return (s - t).norm(); }
	P vec() const { return t - s; }
	P proj(const P& p) { return s + vec().proj(p - s); }
	int iLL(const L& l) const {
		if(sign(vec().det(l.vec()))) return 1;
		if(sign(vec().det(l.s - s))) return 0;
		return -1;
	}
	bool iLS(const L& l) const { return sign(tri(s, t, l.s)) * sign(tri(s, t, l.t)) <= 0; } 
	bool iSS(const L& l) const { return iLS(l) && l.iLS(*this); }
	P pLL(const L& l) const { return s + vec() * (l.s - s).det(l.vec()) / vec().det(l.vec()); }
	double dLP(const P& p) const { return abs(tri(s, t, p)) / vec().norm(); }
	double dSP(const P& p) const {
		if(sign(vec().dot(p - s)) <= 0) return (p - s).norm();
		if(sign(vec().dot(p - t)) >= 0) return (p - t).norm();
		return dLP(p);
	}
};

class C {
public:
	P p;
	double r;
	C() {}
	C(P p_, double r_) : p(p_), r(r_) {}
	C(const C& c) : p(c.p), r(c.r) {}
	C& operator=(const C& c) { p=c.p; r=c.r; return (*this); }
};

// geometry library end

int main() {
	double x1, y1, x2, y2, x3, y3, xp, yp;
	while(scanf("%lf%lf%lf%lf%lf%lf%lf%lf", &x1, &y1, &x2, &y2, &x3, &y3, &xp, &yp) != EOF) {
		P p1(x1, y1);
		P p2(x2, y2);
		P p3(x3, y3);
		P  p(xp, yp);

		double d1 = (p1 - p).det(p2 - p);
		double d2 = (p2 - p).det(p3 - p);
		double d3 = (p3 - p).det(p1 - p);
		if(sign(d1) == sign(d2) && sign(d2) == sign(d3)) {
			printf("YES\n");
		} else {
			printf("NO\n");
		}
	}
}

C++: //外積を使う

#include<iostream>
#include<string>
#include<stdio.h>
using namespace std;

typedef struct{
	double x;
	double y;
}Vector;

Vector sub_vector(Vector a, Vector b){
	Vector ret;
	ret.x = a.x - b.x;
	ret.y = a.y - b.y;
	return ret;
}

int main(){
	
	Vector A, B, C,P;

	while (scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &A.x,&A.y,&B.x,&B.y,&C.x,&C.y,&P.x,&P.y) != EOF){
		Vector AB = sub_vector(B, A);
		Vector BP = sub_vector(P, B);

		Vector BC = sub_vector(C, B);
		Vector CP = sub_vector(P, C);

		Vector CA = sub_vector(A, C);
		Vector AP = sub_vector(P, A);

		double c1 = AB.x * BP.y - AB.y * BP.x;
		double c2 = BC.x * CP.y - BC.y * CP.x;
		double c3 = CA.x * AP.y - CA.y * AP.x;

		if ((c1 > 0 && c2 > 0 && c3 > 0) || (c1 <0 && c2 < 0 && c3 < 0)){
			cout << "YES" << endl;
		}
		else{
			cout << "NO" << endl;
		}

	}

	return 0;
}

C++: #include <iostream>
#include <cmath>
#define mul(s, t) ((s-t)*(s-t))

using namespace std;

int main()
{
	double x[3], y[3], u[2], s, t, v;
	
	while(cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2] >> s >> t){
		bool f = true;
		for(int i = 0; i < 3; i++){
			int p = 0;
			for(int j = 0; j < 3; j++){
				if(i != j){
					u[p++] = sqrt(mul(x[i], x[j])+mul(y[i], y[j]));
				}
			}
			v = sqrt(mul(x[i], s)+mul(y[i], t));
			
			if(u[0] < v || u[1] < v){
				f = false;
				break;
			}
		}
		cout << ((f)? "YES" : "NO") << endl;
	}
	
	return 0;
}

C++: #include<cmath>
#include<algorithm>
#include<iostream>
#include<vector>
#include<climits>
#include<cfloat>

using namespace std;

double EPS = 1e-10;

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

struct point{
  double x, y;
  point(){}
  point(double x,double y) : x(x) , y(y){}

  point operator + (point p){
    return point(add(x,p.x), add(y,p.y));
  }

  point operator - (point p){
    return point(add(x,-p.x), add(y,-p.y));
  }

  point operator * (double d){
    return point(x*d,y*d);
  }

  point operator / (double d){
    return point(x/d,y/d);
  }
  
  bool operator == ( const point &p ) const {
    return fabs(x-p.x) < EPS && fabs(y-p.y) < EPS;
  }
};

double dot(point a, point b) {
  return (a.x * b.x + a.y * b.y);
}

double cross(point a, point b) {
  return (a.x * b.y - a.y * b.x);
}

int is_point_on_line(point a, point b, point c) {
  return cross(b-a, c-a)==0.0 &&
         (dot(b-a, c-a) > -EPS) &&
         (dot(a-b, c-b) > -EPS);
}

int is_intersected_ls(point a1, point a2, point b1, point b2) {

  if(cross(a1-a2,b1-b2)==0){
    return is_point_on_line(a1,a2,b1) || is_point_on_line(a1,a2,b2) 
        || is_point_on_line(b1,b2,a1) || is_point_on_line(b1,b2,a2);
  }
  else {
    return ( cross(a2-a1, b1-a1) * cross(a2-a1, b2-a1) < EPS ) &&
           ( cross(b2-b1, a1-b1) * cross(b2-b1, a2-b1) < EPS );
  }
}

int inside(point p, vector<point> ps){
  point a,b;
  a=b=p;

  b.x=1000000;

  int n=ps.size();
  ps.push_back(ps[0]);

  double ymx=-DBL_MAX,ymn=DBL_MAX;
  for(int i=0;i<n;i++){
    ymx=max(ymx,ps[i].y);
    ymn=min(ymn,ps[i].y);
  }
  if(a.y<=ymn||a.y>=ymx)return 0;
  
  for(int i=0;i<n;i++){
    if(is_point_on_line(ps[i],ps[i+1],p))return 0;
  }
  
  int cnt1=0;
  for(int i=0;i<n;i++)
    if(is_point_on_line(a,b,ps[i]))cnt1++;
  
  int cnt=0;
  for(int i=0;i<n;i++)
    if(is_intersected_ls(ps[i],ps[i+1],a,b))cnt++;
  
  return (cnt-cnt1)%2;
}

int main(void){
 
  point in[4],p;
  vector<point>ps;

  while(cin>>in[1].x>>in[1].y>>in[2].x>>in[2].y>>in[3].x>>in[3].y>>p.x>>p.y){

    ps.clear();

    for(int i=1;i<4;i++)
    ps.push_back(in[i]);

    if(inside(p,ps))cout << "YES" << endl;
    else cout << "NO" << endl;
  }
 
  return 0;
}