2020 AMC 8 第 21 题
先试着解答 2020 AMC 8 第 21 题，然后核对你的答案与视频与精心整理的解答，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 2020 AMC 8 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。
21.
一个棋盘由 个方格组成，阴影格和非阴影格交替排列。下图显示了底行的方格 和顶行的方格 。一个标记放在 上。一步是把标记移到上一行相邻的一个非阴影方格。从 到 有多少条 步路径？（图中显示了一条示例路径。）
A game board consists of squares that alternate between shaded and unshaded. The figure below shows square in the bottom row and square in the top row. A marker is placed at A step consists of moving the marker onto one of the adjoining unshaded squares in the row above. How many -step paths are there from to (The figure shows a sample path.)
小提示：
每一步都上一行，并向左或向右一列
Each move goes one row up and one column left or right
大提示：
每个可达方格的路径数等于下面两个相邻方格路径数之和
Add path counts from the two adjoining squares below
视频讲解：
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文字解答：
每一步都必须向上一行，并向左或向右移动。从 开始逐行计算，每个可达方格的路径数等于下面两个相邻方格路径数之和。
方格 处的计数是 ，所以共有 条路径。
正确答案是 A。
Each move must go up one row and either left or right. Counting row by row from each reachable square gets the sum of the counts from the two adjoining squares below it.
The count at is so there are paths.
Thus, the correct answer is A.
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