2011 AMC 12A 第 22 题
先试着解答 2011 AMC 12A 第 22 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 2011 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。
22.
设 为一个正方形区域, 为整数。若从 内部一点 发出 条射线,可以把 分成 个面积相等的三角形,则称 为 -射线分割点。有多少个点是 -射线分割点但不是 -射线分割点?
Let be a square region and an integer. A point in the interior of is called -ray partitional if there are rays emanating from that divide into triangles of equal area. How many points are -ray partitional but not -ray partitional?
答案:C
解答:
将正方形缩放为 ,并令 射线必须包括通向四个顶点的射线。每个小三角形的面积都是 以底边各段为底的三角形面积之和为 所以这样的三角形有 个。同理,沿上、左、右三边的个数分别是 和
这四个数都必须是正整数。因此 是偶数,并且 反过来,按上述数量把每条边等分,并将分点连接到 ,就会得到 个等面积三角形。因此这些恰好是分割点。
当 时,这些点为 ,其中 共 个。这样的点同时也是 射线分割点,当且仅当对某些整数 有 且 因此 和 都必须是 的倍数。每个坐标有 种选择,所以重合的点有 个。
所以所求数量为
因此,正确答案是 C。
Scale the square to and write The rays must include those through the four vertices. Every small triangle has area The triangles whose bases partition the bottom side together have area so their number is Similarly, the numbers along the top, left, and right sides are and
These four numbers must be positive integers. Hence is even and Conversely, partitioning each side into the indicated number of equal segments and joining the division points to produces equal-area triangles. Thus these are exactly the partitional points.
For the points are with giving Such a point is also -ray partitional exactly when and for integers Thus and must both be multiples of There are choices for each, so the overlap has points.
So the count is
Thus, the correct answer is C.
其他年份的第 22 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 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 12A · 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