题意：给一个多边形柱体的底边形状和体积，求它的高。

题解：叉积求面积，再用体积除以面积。

View Code 
 1 #include<cstdlib>
 2 #include<cmath>
 3 #include<cstdio>
 4 #include<algorithm>
 5 #define max(a,b) (((a)>(b))?(a):(b))
 6 #define min(a,b) (((a)>(b))?(b):(a))
 7 #define sign(x) ((x)>eps?1:((x)<-eps?(-1):(0))) //符号函数
 8 using namespace std;
 9 const int MAXN=10000;
10 const double eps=1e-8,inf=1e50;
11 struct point
12 {
13     double x,y;
14     point(){}
15     point(double _x,double _y){x=_x;y=_y;}
16 };
17 inline double xmult(double x1,double y1,double x2,double y2)
18 {
19     return x1*y2-x2*y1;
20 }
21 inline double getarea(point pg[],int n)
22 {
23     double area=0;
24     pg[n]=pg[0];
25     for(int i=0;i<n;i++)
26         area+=xmult(pg[i].x,pg[i].y,pg[i+1].x,pg[i+1].y);
27     return fabs(area)/2.0;
28 }
29 point pg[MAXN];
30 int main()
31 {
32     int n;
33     while(scanf("%d",&n),n>=3)
34     {
35         for(int i=0;i<n;i++)
36             scanf("%lf%lf",&pg[i].x,&pg[i].y);
37         double v;
38         scanf("%lf",&v);
39         printf("BAR LENGTH: %.2lf\n",v/getarea(pg,n));
40     }
41     return 0;
42 }