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Problem 104

Geometry Difficulty 1.3 Find the answer CEMC Pascal · Canada · 2014

In the diagram, a figure is formed dividing a square into eight identical pieces using its two diagonals and the two lines joining the midpoints of opposite sides, and then drawing a circle in the square as shown.

This figure is reflected in line LL. Which of the following shows the final position of the figure?

(A) (B) [[IMAGE1]] (C) [[IMAGE2]] (D) [[IMAGE3]] (E) [[IMAGE4]]

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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Official solution

Solution 1

The square, its 8 pieces and 2 diagonals are symmetrical about line LL.

Upon reflection, the circle moves from the triangular above LL which is adjacent to and above the second diagonal to the triangular region below LL which is adjacent to and above the second diagonal.

[[IMAGE0]]

Solution 2

We can see that the final position of the figure is given in (D) by first rotating the original figure by 4545^\circ clockwise to obtain:
[[IMAGE1]]
Then, reflecting the figure through the now vertical line, we obtain: [[IMAGE2]] When we reposition the figure and line back to the original position (by rotating counterclockwise by 4545^\circ), we obtain the figure in (D).

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.