三分

  eps = 精度*0.01
  while (r - l > eps) {
    mid = (l + r) / 2;
    lmid = mid - eps;
    rmid = mid + eps;
    if (f(lmid) > f(rmid))
      r = mid; //如果是凹函数 l=mid
    else
      l = mid; //如果是凹函数 r=mid
  }
  printf("%6lf", l);