題目網址: http://www.puzzleup.com/2018/
http://www.puzzleup.com/2018/puzzle/?11
答題時限: １０月11日７ＰＭ－比賽結束（約12月12日）
加分時限: １０月11日７ＰＭ－１０月17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）　　
◆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.
花園裡有X顆樹。任意三樹不共線。你有許多條繩子，顏色有黃色、紅色與藍色。你
要在每顆樹與樹之間綁繩子。三顆樹的繩子會構成三角形，而每出現一個由同色繩
子構成的三角形，便罰你一點。
問若要不被罰任何分數，X的值最大能多大？