数量积 x1x2+y1y2；夹角、模、垂直都有坐标公式。
Poker players sometimes wonder why a four of a kind beats a full house. A poker hand is a random subset of 5 elements…
一副牌五张，四条比葫芦大。不是因为四条"更强",是因为四条更难得——624 种对 3744 种，整整六倍。
