題目網址: http://www.puzzleup.com/2017/
http://www.puzzleup.com/2017/puzzle/?9
答題時限: 10月５日７ＰＭ－比賽結束（約12月27日）
加分時限: 10月５日７ＰＭ－10月10日６：５９ＰＭ
答對可得基本分１００分。答案可上傳５次，每改１次答案從基本分扣２０分。
比賽期間內可隨時上傳答案，加分時限內答對第ｎ天加（６－ｎ）分
另依題目的難易有額外加分（如有８０％的人這題答錯，答對者加８０分）
◆CROSSING POINTS
Place seven points on a plane such that no three are collinear, then connect
every pair with a curve (can be a straight line). On every crossing point
except the initial seven points at most two curves can intersect. What is the
minimum number of these crossing points?
將七個點放在平面上，使得任三點不共線，然後將每一對以曲線連線(可以是直線)。
所有這些線的交點，除了原來的七個點之外，至多有兩條曲線相交。
試問這些交點數目至少有多少？
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