连续 + 两端异号 ⇒ 有零点；个数要结合单调性判。
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。
