2019 AMC 10A Problems/Problem 8

The following problem is from both the 2019 AMC 10A #8 and 2019 AMC 12A #6, so both problems redirect to this page.

Problem

The figure below shows line with a regular, infinite, recurring pattern of squares and line segments.

size(300);
defaultpen(linewidth(0.8));
real r = 0.35;
path P = (0,0)--(0,1)--(1,1)--(1,0), Q = (1,1)--(1+r,1+r);
path Pp = (0,0)--(0,-1)--(1,-1)--(1,0), Qp = (-1,-1)--(-1-r,-1-r);
for(int i=0;i <= 4;i=i+1)
{
draw(shift((4*i,0)) * P);
draw(shift((4*i,0)) * Q);
}
for(int i=1;i <= 4;i=i+1)
{
draw(shift((4*i-2,0)) * Pp);
draw(shift((4*i-1,0)) * Qp);
}
draw((-1,0)--(18.5,0),Arrows(TeXHead));
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How many of the following four kinds of rigid motion transformations of the plane in which this figure is drawn, other than the identity transformation, will transform this figure into itself?

• some rotation around a point of line
• some translation in the direction parallel to line
• the reflection across line
• some reflection across a line perpendicular to line

Solution

Statement one is true. A 180 rotation about the point in between an up-facing square and a down-facing square will yield the same figure. Statement two is also true. A translation to the left or right will place the image onto itself when the figures above and below the line realign (the figure goes on infinitely in both directions). Statement three is false. A reflection across line l will change the up-facing squares to down-facing squares and vice versa. The last statement is also false because it will cause the diagonal lines extending from the squares to switch direction. Thus, only statements are true.

- Violin God - Abhijeet Saha 2.0

See Also

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.