标准方程定圆心半径；一般方程要求 D²+E²-4F>0。
As we have noted, our intuition suggests that the probability of obtaining a head on a single toss of a coin is 1/2. …
让程序模拟扔二十次硬币，结果五个正面、十五个反面。看着像机器坏了。
