2019 AMC 12A 第 8 题
先试着解答 2019 AMC 12A 第 8 题,然后核对你的答案与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2019 AMC 12A 解答,或核对答案。
所有题目均经美国数学协会(MAA)官方合法授权使用。
8.
对于平面上的四条互不相同的直线,恰有 个不同的点同时位于其中两条或更多条直线上。所有可能的 值之和是多少?
For a set of four distinct lines in a plane, there are exactly distinct points that lie on two or more of the lines. What is the sum of all possible values of
答案:D
解答:
当四条直线全部平行或全部共点时,交点数分别为 和 。三条平行线被第四条截时有 个交点。三条共点直线加上一条不经过公共点的直线时有 个交点。一对平行且没有三线共点时有 个交点;四条直线处于一般位置时有 个交点。
数值 不可能。两条直线在 点相交后,第三条若不经过 ,就至少产生一个新交点;第四条不同的直线还必须产生另一个新交点。如果其余直线都经过 则只有一个交点。因此可实现的值为 它们的和为
因此,正确答案是 D。
The values and occur when all four lines are parallel or all four are concurrent. Three parallel lines crossed by a fourth give points. Three concurrent lines together with a fourth line not through their common point give One parallel pair with no three concurrent gives and four lines in general position give
The value is impossible. Once two lines meet at a third line not through creates at least one new intersection; a fourth distinct line must then create another new point. If every remaining line passes through there is only one intersection point instead. Thus the achievable values are whose sum is
Thus, the correct answer is D.
其他年份的第 8 题
1999 AMC 12 · 2000 AMC 12 · 2001 AMC 12 · 2002 AMC 12A · 2002 AMC 12B · 2003 AMC 12A · 2003 AMC 12B · 2004 AMC 12A · 2004 AMC 12B · 2005 AMC 12A · 2005 AMC 12B · 2006 AMC 12A · 2006 AMC 12B · 2007 AMC 12A · 2007 AMC 12B · 2008 AMC 12A · 2008 AMC 12B · 2009 AMC 12A · 2009 AMC 12B · 2010 AMC 12A · 2010 AMC 12B · 2011 AMC 12A · 2011 AMC 12B · 2012 AMC 12A · 2012 AMC 12B · 2013 AMC 12A · 2013 AMC 12B · 2014 AMC 12A · 2014 AMC 12B · 2015 AMC 12A · 2015 AMC 12B · 2016 AMC 12A · 2016 AMC 12B · 2017 AMC 12A · 2017 AMC 12B · 2018 AMC 12A · 2018 AMC 12B · 2019 AMC 12B · 2020 AMC 12A · 2020 AMC 12B · 2021 AMC 12A Spring · 2021 AMC 12B Spring · 2021 AMC 12A Fall · 2021 AMC 12B Fall · 2022 AMC 12A · 2022 AMC 12B · 2023 AMC 12A · 2023 AMC 12B · 2024 AMC 12A · 2024 AMC 12B · 2025 AMC 12A · 2025 AMC 12B