题目链接：hdu 5017 Ellipsoid

题目大意：给定一个面的方程，问在面上距离原点的最小值。

解题思路：三分套三分，先三分x，对于每个x，三分y，求出的最优解作为当前x的值。

#include 
   
     #include 
    
      #include 
     
       #include 
      
        using namespace std; const double INF = 10000; const double eps = 1e-9; double a, b, c, d, e, f; double get(double A, double B, double C) { if (B * B - 4 * A * C < eps) return INF; return (sqrt(B * B - 4 * A * C) - B) / (2 * A); } double func (double x, double y) { double z = get(c, e * x + d * y, a * x * x + b * y * y + f * x * y - 1); return x * x + y * y + z * z; } double search (double x) { double l = -INF, r = INF; for (int i = 0; i < 200; i++) { double ll = l + (r - l) / 3; double rr = r - (r - l) / 3; if (func(x, ll) > func(x, rr)) l = ll; else r = rr; } return func(x, l); } double solve (double l, double r) { for (int i = 0; i < 200; i++) { double ll = l + (r - l) / 3; double rr = r - (r - l) / 3; if (search(ll) > search(rr)) l = ll; else r = rr; } return sqrt(search(l)); } int main () { while (scanf("%lf%lf%lf%lf%lf%lf", &a, &b, &c, &d, &e, &f) == 6) { printf("%.5lf\n", solve(-INF, INF)); } return 0; }