数列是离散型函数 an=f(n)；通项与递推是两条路。
A casino offers a game of chance. A fair coin is tossed until the first time it comes up heads. If this happens on th…
掷硬币，第一次正面就停，第 k 次才正面拿 2^k 块钱。这个赌局的期望收益是无穷大——可没人愿意花一万块玩它。
