Maths Olympiad Prep

Track / Stage 6 / 241 of 400 #1241 of 1964

Problem 1241

National olympiad, first round
Geometry Difficulty 6.3 Find the answer

In the square, the midpoints of the two sides were marked and the segments shown in the figure on the left were drawn. Which of the shaded quadrilaterals has the largest area?
[img]https://cdn.artofproblemsolving.com/attachments/d/f/2be7bcda3fa04943687de9e043bd8baf40c98c.png[/img]

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

1. Identify the midpoints and draw the segments:
- Let the square be ABCDABCD with side length ss.
- Mark the midpoints MM and NN of sides ABAB and BCBC respectively.
- Draw segments AMAM, BNBN, CMCM, and DNDN.

2. Analyze the quadrilaterals formed:
- The segments divide the square into four quadrilaterals.
- The quadrilateral with more dense cross-hatching is formed by the segments AMAM and DNDN.
- The quadrilateral with less dense cross-hatching is formed by the segments BNBN and CMCM.
- The middle quadrilateral is formed by the intersection of all four segments.

3. Calculate the areas of the quadrilaterals:
- The area of the entire square is s2s^2.
- Each of the four quadrilaterals is a right triangle with legs of length s2\frac{s}{2}.
- The area of each right triangle is 12×s2×s2=s28\frac{1}{2} \times \frac{s}{2} \times \frac{s}{2} = \frac{s^2}{8}.

4. Combine the areas:
- The middle quadrilateral is formed by the intersection of the four right triangles.
- The area of the middle quadrilateral is s24\frac{s^2}{4} (since it is half of the area of the square minus the areas of the four right triangles).

5. Compare the areas:
- The quadrilateral with more dense cross-hatching is formed by combining two right triangles.
- The area of this quadrilateral is 2×s28=s242 \times \frac{s^2}{8} = \frac{s^2}{4}.
- The quadrilateral with less dense cross-hatching is also formed by combining two right triangles.
- The area of this quadrilateral is also s24\frac{s^2}{4}.

6. Conclusion:
- Since the middle quadrilateral has an area of s24\frac{s^2}{4} and the quadrilateral with more dense cross-hatching has an area of s24\frac{s^2}{4}, the quadrilateral with more dense cross-hatching has the largest area.

The final answer is the quadrilateral with more dense cross-hatching.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.