Go: package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
	"strings"
)

var scanner = bufio.NewScanner(os.Stdin)

func main() {
	for scanner.Scan() {
		buf0 := strings.Split(scanner.Text(), " ")
		input0 := make([]int, 2)
		for i := range input0 {
			input0[i], _ = strconv.Atoi(buf0[i])
		}
		if input0[0] == 0 && input0[1] == 0 {
			break
		}
		a, b := input0[0], input0[1]
		scanner.Scan()
		buf1 := strings.Split(scanner.Text(), " ")
		input1 := make([]int, 2)
		for i := range input1 {
			input1[i], _ = strconv.Atoi(buf1[i])
		}
		c, d := input1[0], input1[1]
		scanner.Scan()
		buf2 := strings.Split(scanner.Text(), " ")
		input2 := make([]int, 2)
		for i := range input2 {
			input2[i], _ = strconv.Atoi(buf2[i])
		}
		e, f := input2[0], input2[1]
		scanner.Scan()
		buf3 := strings.Split(scanner.Text(), " ")
		input3 := make([]int, 2)
		for i := range input3 {
			input3[i], _ = strconv.Atoi(buf3[i])
		}
		g, h := input3[0], input3[1]
		scanner.Scan()
		r, _ := strconv.Atoi(scanner.Text())
		if (a-g)*(a-g)+(b-h)*(b-h) <= r*r && (c-g)*(c-g)+(d-h)*(d-h) <= r*r && (e-g)*(e-g)+(f-h)*(f-h) <= r*r {
			fmt.Println("b")
		} else if dist(a, b, c, d, g, h) > float64(r)-0.000001 && dist(a, b, e, f, g, h) > float64(r)-0.000001 && dist(e, f, c, d, g, h) > float64(r)-0.000001 && inclusive(a, b, c, d, e, f, g, h) {
			fmt.Println("a")
		} else if CL_intersect(a, b, c, d, g, h, r) || CL_intersect(a, b, e, f, g, h, r) || CL_intersect(e, f, c, d, g, h, r) {
			fmt.Println("c")
		} else {
			fmt.Println("d")
		}
	}
}

func CL_intersect(ax, ay, bx, by, cx, cy, r int) bool {
	acx := cx - ax
	acy := cy - ay
	bcx := cx - bx
	bcy := cy - by
	abx := bx - ax
	aby := by - ay
	if acx*acx+acy*acy >= r*r || bcx*bcx+bcy*bcy >= r*r {
		if abx*acx+aby*acy < 0 {
			if acx*acx+acy*acy <= r*r {
				return true
			}
		} else {
			if abx*acx+aby*acy > abx*abx+aby*aby {
				if bcx*bcx+bcy*bcy <= r*r {
					return true
				}
			} else if acx*acx+acy*acy-(abx*acx+aby*acy)*(abx*acx+aby*acy)/(abx*abx+aby*aby) <= r*r {
				return true
			}
		}
	}
	return false
}

func inclusive(ax, ay, bx, by, cx, cy, px, py int) bool {
	ok1 := true
	ok2 := true
	if (bx-ax)*(py-ay)-(px-ax)*(by-ay) < 0 {
		ok1 = false
	}
	if (bx-ax)*(py-ay)-(px-ax)*(by-ay) > 0 {
		ok2 = false
	}
	if (cx-bx)*(py-by)-(px-bx)*(cy-by) < 0 {
		ok1 = false
	}
	if (cx-bx)*(py-by)-(px-bx)*(cy-by) > 0 {
		ok2 = false
	}
	if (ax-cx)*(py-cy)-(px-cx)*(ay-cy) < 0 {
		ok1 = false
	}
	if (ax-cx)*(py-cy)-(px-cx)*(ay-cy) > 0 {
		ok2 = false
	}
	return ok1 || ok2
}

func dist(ax, ay, bx, by, px, py int) float64 {
	dx := float64(bx - ax)
	dy := float64(by - ay)
	A := float64(dx*dx + dy*dy)
	B := dx*float64(ax-px) + dy*float64(ay-py)
	t := -B / A
	if t < 0 {
		t = 0
	}
	if t > 1 {
		t = 1
	}
	tx := float64(ax) + dx*t
	ty := float64(ay) + dy*t
	return math.Sqrt((float64(px)-tx)*(float64(px)-tx) + (float64(py)-ty)*(float64(py)-ty))
}

C++: #include <iostream>
#include <cmath>
#include <vector>
#include <algorithm>
#include <complex>

#define REP(i,k,n) for(int i=k;i<n;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define EPS 1e-10

using namespace std;

typedef complex<double> Po;

struct Point 
{
    double x, y;
    Point(double x=0, double y=0) : x(x), y(y) {}
    
	double distance (const Point &o) const 
	{
        return sqrt((x - o.x) * (x - o.x) + (y - o.y) * (y - o.y));
    }

    Point operator+(const Point &o) const 
	{
        return Point(x+o.x, y+o.y);
    }
 
    Point operator-(const Point &o) const 
	{
        return Point(x-o.x, y-o.y);
    }
 	
    Point operator*(const double m) const 
	{
   	   return Point(x*m, y*m);
    }

    Point operator/(const double d) const 
	{
       return Point(x/d, y/d);
    }

    double cross(const Point &o) const 
	{
       return x * o.y - y * o.x;
    }
 
    double dot(const Point &o) const 
	{
        return x * o.x + y * o.y;
    }
 
    double atan() const 
	{
        return atan2(y, x);
    }

    double norm() const 
	{
        return sqrt(dot(*this));
    }

	double area_abs(const Point &a,const Point &b) const
	{
		double t = (a.x - x) * (b.y - y);
		double t2 = (a.y - y) * (b.x - x);
		double areas = abs(t-t2);
		return areas/2;
	}

	//符号付き面積
	double area(const Point &a,const Point &b)
	{
		return ((a.x - x) * (b.y - y)) + ((b.x - x) * (y - a.y));
	}
	//線分abが自身の含まれているのかどうか判断する
	int between(const Point &a,const Point &b)
	{
		if(area(a,b) != 0) return 0;

		if(a.x != b.x)
		{
			return ((a.x <= x) && (x <= b.x) || (a.x >= x) && (x >= b.x));
		}
		else
		{
			return ((a.y <= y) && (y <= b.y) || (a.y >= y) && (y >= b.y));
		}
	}

	double distance_seg(const Point& a,const Point& b)
	{
		if((b-a).dot(*this-a) < EPS) 
		{
			return (*this-a).norm();
		}
		if((a-b).dot(*this-b) < EPS)
		{
			return (*this-b).norm();
		}
		return abs((b-a).cross(*this-a)) / (b-a).norm();
	}

	//点a,b,cからなる三角形の中に自身が含まれているかどうか
	bool hitPolygon(const Point& a,const Point& b,const Point& c)
	{
		double t = (b-a).cross(*this-b);
		double t2 = (c-b).cross(*this-c);
		double t3 = (a-c).cross(*this-a);

		if((t > 0 && t2 > 0 && t3 > 0) || ( t < 0 && t2 < 0 && t3 < 0))
		{
			return 1;
		}

		return 0;
	}
};

int main()
{
	double a,b;
	while(cin >> a >> b)
	{
		if(a == 0 && b == 0) break;
		
		vector<Point> t;
		t.push_back(Point(a,b));

		rep(i,2)
		{
			cin >> a >> b;
			t.push_back(Point(a,b));
		}

		double r;
		cin >> a >> b >> r;
		Point c(a,b);

		int cnt = 0;

		rep(i,3)
		{
			if(t[i].distance(c) >= r)
			{
				cnt++;
			}
		}

		if(cnt == 0) cout << "b" << endl;
		else if(cnt == 1 || cnt == 2) cout << "c" << endl;
		else
		{
			bool flag = false;

			if(c.distance_seg(t[0],t[1]) < r) flag = true;
			if(c.distance_seg(t[1],t[2]) < r) flag = true;
			if(c.distance_seg(t[2],t[0]) < r) flag = true;
			
			if(flag) cout << "c" << endl;
			else
			{
				if(c.hitPolygon(t[0],t[1],t[2]))
				{
					cout << "a" << endl;
				}
				else
				{
					bool flag2 = false;

					if(c.distance_seg(t[0],t[1]) == r) flag2 = true;
					if(c.distance_seg(t[1],t[2]) == r) flag2 = true;
					if(c.distance_seg(t[2],t[0]) == r) flag2 = true;

					if(flag2) cout << "c" << endl;
					else cout << "d" << endl;
				}
			}
		}
	}
	return 0;
}

C++: #include<stdio.h>
#include<algorithm>
using namespace std;
bool CL_intersect(int ax,int ay,int bx,int by,int cx,int cy,int r){
	int acx=cx-ax;
	int acy=cy-ay;
	int bcx=cx-bx;
	int bcy=cy-by;
	int abx=bx-ax;
	int aby=by-ay;
	if(acx*acx+acy*acy>=r*r||bcx*bcx+bcy*bcy>=r*r){
		if(abx*acx+aby*acy<0){
			if(acx*acx+acy*acy<=r*r)return true;
		}else{
			if(abx*acx+aby*acy>abx*abx+aby*aby){
				if(bcx*bcx+bcy*bcy<=r*r)return true;
			}else if(acx*acx+acy*acy-(long long)(abx*acx+aby*acy)*(abx*acx+aby*acy)/(abx*abx+aby*aby)<=r*r)return true;
		}
	}
	return false;
}
bool inclusive(int ax,int ay,int bx,int by,int cx,int cy,int px,int py){
	bool ok1=true;
	bool ok2=true;
	if((bx-ax)*(py-ay)-(px-ax)*(by-ay)<0)ok1=false;
	else ok2=false;
	if((cx-bx)*(py-by)-(px-bx)*(cy-by)<0)ok1=false;
	else ok2=false;
	if((ax-cx)*(py-cy)-(px-cx)*(ay-cy)<0)ok1=false;
	else ok2=false;
	return ok1||ok2;
}
int main(){
	int a,b,c,d,e,f,g,h,r;
	while(scanf("%d%d",&a,&b),a||b){
		scanf("%d%d%d%d%d%d%d",&c,&d,&e,&f,&g,&h,&r);
		if((a-g)*(a-g)+(b-h)*(b-h)<r*r&&(c-g)*(c-g)+(d-h)*(d-h)<r*r&&(e-g)*(e-g)+(f-h)*(f-h)<r*r)printf("b\n");
		else if(CL_intersect(a,b,c,d,g,h,r)||CL_intersect(a,b,e,f,g,h,r)||CL_intersect(e,f,c,d,g,h,r))printf("c\n");
		else if(inclusive(a,b,c,d,e,f,g,h))printf("a\n");
		else printf("d\n");
	}
}