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Geometry Difficulty 7.1 National olympiad, round 2 Find the answer

Let MM be the intersection of diagonals of the convex quadrilateral ABCDABCD, where m(AMB^)=60m(\widehat{AMB})=60^\circ. Let the points O1O_1, O2O_2, O3O_3, O4O_4 be the circumcenters of the triangles ABMABM, BCMBCM, CDMCDM, DAMDAM, respectively. What is Area(ABCD)/Area(O1O2O3O4)Area(ABCD)/Area(O_1O_2O_3O_4)?

Pick one

Solution

1. **Calculate the area of quadrilateral ABCDABCD:**
- The area of ABCDABCD can be divided into the sum of the areas of triangles ABMABM, BCMBCM, CDMCDM, and DAMDAM.
- The area of each triangle can be calculated using the formula for the area of a triangle:
Area=12×base×height×sin(angle) \text{Area} = \frac{1}{2} \times \text{base} \times \text{height} \times \sin(\text{angle})
- Given m(AMB^)=60m(\widehat{AMB}) = 60^\circ, we have:
[ABM]=12×AM×BM×sin(60)=12×AM×BM×32=AM×BM×34 [ABM] = \frac{1}{2} \times AM \times BM \times \sin(60^\circ) = \frac{1}{2} \times AM \times BM \times \frac{\sqrt{3}}{2} = \frac{AM \times BM \times \sqrt{3}}{4}
- Similarly, we can calculate the areas of the other triangles:
[BCM]=12×BM×CM×sin(60)=BM×CM×34 [BCM] = \frac{1}{2} \times BM \times CM \times \sin(60^\circ) = \frac{BM \times CM \times \sqrt{3}}{4}
[CDM]=12×CM×DM×sin(60)=CM×DM×34 [CDM] = \frac{1}{2} \times CM \times DM \times \sin(60^\circ) = \frac{CM \times DM \times \sqrt{3}}{4}
[DAM]=12×DM×AM×sin(60)=DM×AM×34 [DAM] = \frac{1}{2} \times DM \times AM \times \sin(60^\circ) = \frac{DM \times AM \times \sqrt{3}}{4}
- Summing these areas, we get:
[ABCD]=[ABM]+[BCM]+[CDM]+[DAM]=AM×BM×34+BM×CM×34+CM×DM×34+DM×AM×34 [ABCD] = [ABM] + [BCM] + [CDM] + [DAM] = \frac{AM \times BM \times \sqrt{3}}{4} + \frac{BM \times CM \times \sqrt{3}}{4} + \frac{CM \times DM \times \sqrt{3}}{4} + \frac{DM \times AM \times \sqrt{3}}{4}
- Since MM is the intersection of the diagonals, we can express the area in terms of the diagonals ACAC and BDBD:
[ABCD]=AC×BD×34 [ABCD] = \frac{AC \times BD \times \sqrt{3}}{4}

2. **Calculate the area of quadrilateral O1O2O3O4O_1O_2O_3O_4:**
- The points O1,O2,O3,O4O_1, O_2, O_3, O_4 are the circumcenters of triangles ABMABM, BCMBCM, CDMCDM, and DAMDAM, respectively.
- The quadrilateral O1O2O3O4O_1O_2O_3O_4 is a parallelogram because the perpendicular bisectors of the sides of the triangles intersect at right angles.
- The area of the parallelogram O1O2O3O4O_1O_2O_3O_4 can be calculated using the formula for the area of a parallelogram:
Area=base×height \text{Area} = \text{base} \times \text{height}
- The length of the sides of the parallelogram can be related to the diagonals ACAC and BDBD of the original quadrilateral ABCDABCD:
O1O2=O3O4=BD2sin(60)=BD3 O_1O_2 = O_3O_4 = \frac{BD}{2 \sin(60^\circ)} = \frac{BD}{\sqrt{3}}
O1O4=O2O3=AC2sin(60)=AC3 O_1O_4 = O_2O_3 = \frac{AC}{2 \sin(60^\circ)} = \frac{AC}{\sqrt{3}}
- Therefore, the area of O1O2O3O4O_1O_2O_3O_4 is:
[O1O2O3O4]=AC3×BD3=AC×BD3 [O_1O_2O_3O_4] = \frac{AC}{\sqrt{3}} \times \frac{BD}{\sqrt{3}} = \frac{AC \times BD}{3}

3. Calculate the ratio of the areas:
- The ratio of the area of ABCDABCD to the area of O1O2O3O4O_1O_2O_3O_4 is:
[ABCD][O1O2O3O4]=AC×BD×34AC×BD3=3413=34×3=334=32 \frac{[ABCD]}{[O_1O_2O_3O_4]} = \frac{\frac{AC \times BD \times \sqrt{3}}{4}}{\frac{AC \times BD}{3}} = \frac{\frac{\sqrt{3}}{4}}{\frac{1}{3}} = \frac{\sqrt{3}}{4} \times 3 = \frac{3\sqrt{3}}{4} = \frac{3}{2}

The final answer is 32\boxed{\frac{3}{2}}

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