2001 AMC 10 第 5 题

Question

下图中的十二个五连方中，有多少个至少有一条对称轴？

How many of the twelve pentominoes pictured below have at least one line of symmetry?

3

4

5

6

7

对称轴会把图形折叠后与自身完全重合。

A line of symmetry folds the shape exactly onto itself

分别检查十二个图形是否有水平、竖直或斜向的对称轴。

Check each of the twelve shapes for a horizontal, vertical, or diagonal fold line

逐一检查这些五连方的反射对称性，恰好有六个至少有一条对称轴：直条形、十字形、

T

形、

U

形、

V

形和

W

形。其余六个都没有对称轴。

所以正确答案是 D。

Checking each pentomino for a reflection line, exactly six have at least one: the straight bar, the plus, the

T

-shape, the

U

-shape, the

V

-shape, and the

W

-shape. The remaining six have no line of symmetry.

Thus, the correct answer is D.

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