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 <bits/stdc++.h>
using namespace std ;

#define pb(n) push_back(n)
#define fi first
#define se second
#define X real()
#define Y imag()
#define all(r) (r).begin(),(r).end()
#define gsort(st,en) sort((st),(en),greater<int>())
#define vmax(ary) *max_element(all(ary))
#define vmin(ary) *min_element(all(ary))
#define debug(x) cout<<#x<<": "<<x<<endl
#define fcout(n) cout<<fixed<<setprecision((n))
#define scout(n) cout<<setw(n)
#define vary(type,name,size,init) vector< type> name(size,init)

#define rep(i,n) for(int i = 0; i < (int)(n);++i)
#define REP(i,a,b) for(int i = (a);i < (int)(b);++i)
#define repi(it,array) for(auto it = array.begin(),end = array.end(); it != end;++it)
#define repa(n,array) for(auto &n :(array))

using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using dict = map<string,int>;
using comd = complex<double>;
using pii = pair<int,int> ;

constexpr int imax = ((1<<30)-1)*2+1 ;
constexpr int inf = 100000000;
constexpr double PI = acos(-1.0) ;
double eps = 1e-10 ;
const int dy[] = {-1,0,1,0};
const int dx[] = {0,-1,0,1};

inline bool value(int x,int y,int w,int h){
  return (x >= 0 && x < w && y >= 0 && y < h);
}

double CalcDist(comd p1, comd p2){
  return sqrt(pow(p1.X - p2.X,2.0) + pow(p1.Y -p2.Y,2.0));
}

template<typename T>
void Unique(vector<T> &v){
  sort(all(v));
  v.erase(all(v),v.end());
}

template<typename T>
T ston(string& str, T n){
  istringstream sin(str) ;
  T num ;
  sin >> num ;
  return num ;
}

void Ans(bool f){
  if(f) cout << "YES"<<endl;
  else cout << "NO"<<endl;
}

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 P{
  double x,y;
  P(){}
  P(double _x,double _y):x(_x),y(_y){}
  P operator + (P p){
    return P(add(x,p.x) ,add(y,p.y));
  }
  P operator - (P p){
    return P(add(x,-p.x) ,add(y,-p.y));
  }
  P operator * (double d){
    return P(x * d,y * d);
  }
  double dot(P p){
    return add(x * p.x,y*p.y);
  }
  double det(P p){
    return add(x*p.y,-y*p.x);
  }
  double dist2(P p){
    return ((*this-p).x * (*this-p).x) + ((*this-p).y * (*this-p).y);
  }
  double GetSlope(P &p){
    P tmp = *this - p;
    return tmp.y/tmp.x;
  }
};
//???????????????????????¨?????????
bool on_seg(P p1,P p2,P q){
  return (p1-q).det(p2-q) == 0 && (p1-q).dot(p2-q) <= 0;
}
int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  double a,b,c,d,e,f,xp,yp;
  while(cin >> a >> b >> c >> d >> e >> f >> xp >> yp){
    bool flag = true;
    double acx = c-a,abx = e-a,acy = d-b,aby = f-b,apx = xp-a,apy = yp - b;
    P p1 = P(a,b),p2 = P(c,d),p3 = P(e,f),p4 = P(xp,yp);
    double t,s;
    if(acx == 0){
      s = apx/ abx;
      t = (apy - s * aby)/acy;
    }
    else if(acy == 0){
      s = apy/aby;
      t = (apx - s * abx)/acx;
    }
    else if(abx == 0){
      t = apx / acx;
      s = (apy- t * acy)/aby;
    }
    else if(aby == 0){
      t = apy/acy;
      s = (apx - t * acx)/abx;
    }
    else{
//       t = (abx * apy - apx * aby)/(acy - acx*aby);
      t = (apy * abx - aby * apx)/(acy * (abx - aby) - (acx - acy));
//       s = (apx - t * acx)/abx;
      s = ((apx - apy) - t * (acx - acy))/(abx - aby);
    }
    if(on_seg(p1,p2,p4) || on_seg(p1,p3,p4) || on_seg(p2,p3,p4)){
      flag = false;
    }
    else{
      flag = 0 < t+s && t+s < 1 && s > 0 && t > 0;
    }
    Ans(flag);
  }
  return 0;
}

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

const double EPS = 1e-8;
const double INF = 1e12;
typedef complex<double> P;
namespace std{
	bool operator < (const P& a, const P& b) {
		return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
	}
}
double cross(const P& a, const P& b) {
	return imag(conj(a)*b);
}
double dot(const P& a, const P& b) {
	return real(conj(a)*b);
}
bool in_triangle(const P& p1,const P& p2,const P& p3,const P& p){
	return cross(p2-p1,p-p1)>0 && cross(p3-p2,p-p2)>0 && cross(p1-p3,p-p3)>0;
}
int main(){
	double x1,y1,x2,y2,x3,y3,x,y;
	for(;~scanf("%lf%lf%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3,&x,&y);){
		P p1(x1,y1),p2(x2,y2),p3(x3,y3),p(x,y);
		bool in=in_triangle(p1,p2,p3,p)||in_triangle(p2,p1,p3,p);
		puts(in?"YES":"NO");
	}
}

C++: #include<iostream>
using namespace std;
int main()
{
double x;
double x1,y1,x2,y2,x3,y3,xp,yp;
int S=0;
while(cin>>x1>>y1>>x2>>y2>>x3>>y3>>xp>>yp)
{
	if(y1<y2&&y3<y2)
	{
	for(double i=x1;i<=x3;i+=0.0001)
{
x=i;
if((y1-y2)/(x1-x2)*x+y1-(y1-y2)/(x1-x2)*x1>(y1-y2)/(x1-x2)*xp+y1-(y1-y2)/(x1-x2)*x1&&(y2-y3)/(x2-x3)*x+y2-(y2-y3)/(x2-x3)*x2>(y2-y3)/(x2-x3)*xp+y2-(y2-y3)/(x2-x3)*x2&&(y1-y3)/(x1-x3)*x+y1-(y1-y3)/(x1-x3)*x2<(y1-y3)/(x1-x3)*xp+y1-(y1-y3)/(x1-x3)*x1)
{
	S++;
cout<<"YES"<<endl;
break;	
}
}
}
else if(y1>y2&&y1>y1)
{
for(double i=x1;i<=x3;i+=0.0001)
{x=i;
if((y1-y2)/(x1-x2)*x+y1-(y1-y2)/(x1-x2)*x1>(y1-y2)/(x1-x2)*xp+y1-(y1-y2)/(x1-x2)*x1&&(y2-y3)/(x2-x3)*x+y2-(y2-y3)/(x2-x3)*x2<(y2-y3)/(x2-x3)*xp+y2-(y2-y3)/(x2-x3)*x2&&(y1-y3)/(x1-x3)*x+y1-(y1-y3)/(x1-x3)*x2>(y1-y3)/(x1-x3)*xp+y1-(y1-y3)/(x1-x3)*x1)
{
	S++;
cout<<"YES"<<endl;
break;	
	}	
	}	
}
else if(y3>y1&&y3>y2)
{
	for(double i=x1;i<=x3;i+=0.0001)
{x=i;
if((y1-y2)/(x1-x2)*x+y1-(y1-y2)/(x1-x2)*x1<(y1-y2)/(x1-x2)*xp+y1-(y1-y2)/(x1-x2)*x1&&(y2-y3)/(x2-x3)*x+y2-(y2-y3)/(x2-x3)*x2>(y2-y3)/(x2-x3)*xp+y2-(y2-y3)/(x2-x3)*x2&&(y1-y3)/(x1-x3)*x+y1-(y1-y3)/(x1-x3)*x2>(y1-y3)/(x1-x3)*xp+y1-(y1-y3)/(x1-x3)*x1)
{
	S++;
cout<<"YES"<<endl;
break;
}
}
}

{
if(S<1)
{
 	cout<<"NO"<<endl;
 }
}
}
return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<iostream>
#include<cstdlib>
#include<cmath>
#include<algorithm>
using namespace std;

double GetLen(double x1,double y1,double x2,double y2){
  return sqrt((y2-y1)*(y2-y1)+(x2-x1)*(x2-x1));
}
double GetAn1(double x1,double y1,double x2,double y2){
  return atan((y2-y1)/(x2-x1));
}
double GetAn2(double a,double b,double c){
  return  acos((b*b+c*c-a*a)/(2*b*c));
}
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){
    double S,S2;
    double a,b,c,an;
    double cl1,cl2,cl3,anc;

    if( (xp==x1 && yp==y1) || (xp==x2 && yp==y2) || (xp==x3 && yp==y3)){
      printf("YES\n");
      continue;
    }

    a=GetLen(x1,y1,x2,y2);
    b=GetLen(x2,y2,x3,y3);
    c=GetLen(x3,y3,x1,y1);
    an=GetAn2(a,b,c);
    S=0.5*b*c*sin(an);

    cl1=GetLen(x1,y1,xp,yp);
    cl2=GetLen(x2,y2,xp,yp);
    cl3=GetLen(x3,y3,xp,yp);

    anc=GetAn2(a,cl1,cl2);
    S2=0.5*cl1*cl2*sin(anc);

    anc=GetAn2(b,cl2,cl3);
    S2+=0.5*cl2*cl3*sin(anc);

    anc=GetAn2(c,cl3,cl1);
    S2+=0.5*cl3*cl1*sin(anc);
    long long int Si,Si2;
    Si=S*10000000;
    Si2=S2*10000000;
    //  printf("%f %f %f %f %f %f",a,b,c,an*(180/M_PI),S,S2);
    if(Si2<=Si)
      printf("YES\n");
    else printf("NO\n");
  }
}