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Geometry Difficulty 6.7 National olympiad Prove it Slovenia

Let ABC\triangle ABC be an equilateral triangle with the side of 2020 units. Vid divides this triangle into 400400 smaller equilateral triangles with the sides of 11 unit. Eva then picks 44 of the vertices of these smaller triangles. The vertices lie inside the triangle ABCABC and form a parallelogram with sides parallel to the sides of the triangle ABCABC. There are exactly 4646 smaller triangles that have at least one point in common with the sides of this parallelogram. Find all possible values for the area of this parallelogram.

Figure 1

Solution

Denote the sides of the parallelogram by aa and bb. We may assume that aba \ge b. The small triangles which share at least one point with the sides of the parallelogram are marked in the figure. We can find their number by subtracting 22 triangles in the corners and the number of the triangles inside the white parallelogram from the number of all triangles in the enlarged parallelogram.

Figure 2

When b2b \le 2 there is no white parallelogram. If b=1b = 1, then the number of the small triangles is equal to the number of the triangles inside a parallelogram of size (a+2)×3(a + 2) \times 3, minus 22. There are 23(a+2)2 \cdot 3(a + 2) of the triangles in the parallelogram, so 6(a+2)2=466(a + 2) - 2 = 46. This implies a=6a = 6.

If b=2b = 2, then there are 2(a+2)42=462 \cdot (a + 2) \cdot 4 - 2 = 46 small triangles and this implies a=8a = 8.

Let b3b \ge 3. The enlarged parallelogram contains 2(a+2)(b+2)2(a + 2)(b + 2) small triangles and the white parallelogram contains 2(a2)(b2)2(a - 2)(b - 2) small triangles. We get
2(a+2)(b+2)2(a2)(b2)2=46, 2(a + 2)(b + 2) - 2(a - 2)(b - 2) - 2 = 46,
and this implies a+b=6a + b = 6. Since ab3a \ge b \ge 3, this is only possible when a=b=3a = b = 3.

Let us find the areas of these parallelograms. Since the acute angle in these parallelograms measures 6060^\circ, the areas are equal to 3ab2\frac{\sqrt{3}ab}{2}.

When a=6a = 6, b=1b = 1 we get 333\sqrt{3}.

When a=b=3a = b = 3 we get 932\frac{9\sqrt{3}}{2}.

When a=4a = 4, b=2b = 2 we get 434\sqrt{3}.

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