题目网址: http://www.puzzleup.com/2018/
http://www.puzzleup.com/2018/puzzle/?11
答题时限: １０月11日７ＰＭ－比赛结束（约12月12日）
加分时限: １０月11日７ＰＭ－１０月17日６：５９ＰＭ
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◆CORDED TRIANGLES
There are X trees in a garden. No three of them are collinear. You have
many cords colored yellow, red or blue and you will connect every tree
pair with a cord. Cords connecting three trees form triangles and you
will receive a penalty point for each triangle that consists of three
cords of the same color.
What is the maximum value of X that allows you to place the cords without
receiving any penalty points.
花园里有X颗树。任意三树不共线。你有许多条绳子，颜色有黄色、红色与蓝色。你
要在每颗树与树之间绑绳子。三颗树的绳子会构成三角形，而每出现一个由同色绳
子构成的三角形，便罚你一点。
问若要不被罚任何分数，X的值最大能多大？