description：
某个地方被Mississippi河腐蚀每年都会有50square miles，而且总共腐蚀是个半圆，求给定一个点什么时候会被腐蚀
solution：
求该点到（0，0）的长度r，求以r为半径半圆的面积area，除以50即可得到答案
1. 一个难点是浮点型精度的处理，但是好像多想了，题目给出HINT：No property will appear exactly on the semicircle boundary: it will either be inside or outside.
1 #include <stdio.h>
2
3 #define PI 3.1415926535
4 #define EPS 1e-6
5
6 int main()
7 {
8 int cases=0,i=0;
9 double year=0;
10 double area=0,x=0,y=0;
11
12 scanf ("%d",&cases);
13
14 for (i=1; i<=cases; i++)
15 {
16 scanf ("%lf %lf",&x,&y);
17 area = (x*x+y*y)*PI/2;
18
19 year = area / 50;
20 printf ("Property %d: This property will begin eroding in year %d.\n",
21 i,(int)(year-EPS)+1);
22 }
23
24 printf("END OF OUTPUT.\n");
25
26 return 0;
27 }