POJ 1375

可以通过顶角角度来计算切线与坐标轴的坐标

开始还以为有公式可以算，谁知道原来是解二元一次方程，靠。。

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>

using namespace std;
const int MAX=1050;

struct point{
	double x;
	int flag;
	point(){ flag=0;}
}p[MAX];

double dist(double x1,double y1,double x2,double y2){
	return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}

bool cmp(point A, point B){
	if(A.x<B.x)return true;
	else if(A.x==B.x){
		if(A.flag<B.flag) return true;
	}
	return false;
}

int main(){
	double lx,ly; int n; double x,y,r;
	while(scanf("%d",&n)!=EOF){
		if(n==0) break;
		scanf("%lf%lf",&lx,&ly);
		for(int i=0;i<n;i++){
			scanf("%lf%lf%lf",&x,&y,&r);
			double dis=dist(lx,ly,x,y);
			double Agle=asin(r/dis); double Bgle=asin((lx-x)/dis);
			double bx=lx-ly*tan(Agle+Bgle); 
			double ex=lx-ly*tan(Bgle-Agle);
			p[2*i].x=bx; p[2*i].flag=-1;
			p[2*i+1].x=ex; p[2*i+1].flag=1;
		}
		sort(p,p+2*n,cmp);
		int now=0; double L;
		for(int i=0;i<2*n;i++){
			if(now==0){
			L=p[i].x;
			}
			now+=p[i].flag;
			if(now==0)
			printf("%0.2f %0.2f\n",L,p[i].x);
		}
		printf("\n");
	}
	return 0;
}