GeometryDifficulty 7.1National olympiad, round 2Find the answer
Let M be the intersection of diagonals of the convex quadrilateral ABCD, where m(AMB)=60∘. Let the points O1, O2, O3, O4 be the circumcenters of the triangles ABM, BCM, CDM, DAM, respectively. What is Area(ABCD)/Area(O1O2O3O4)?
Pick one
Solution
1. **Calculate the area of quadrilateral ABCD:** - The area of ABCD can be divided into the sum of the areas of triangles ABM, BCM, CDM, and DAM. - The area of each triangle can be calculated using the formula for the area of a triangle: Area=21×base×height×sin(angle) - Given m(AMB)=60∘, we have: [ABM]=21×AM×BM×sin(60∘)=21×AM×BM×23=4AM×BM×3 - Similarly, we can calculate the areas of the other triangles: [BCM]=21×BM×CM×sin(60∘)=4BM×CM×3 [CDM]=21×CM×DM×sin(60∘)=4CM×DM×3 [DAM]=21×DM×AM×sin(60∘)=4DM×AM×3 - Summing these areas, we get: [ABCD]=[ABM]+[BCM]+[CDM]+[DAM]=4AM×BM×3+4BM×CM×3+4CM×DM×3+4DM×AM×3 - Since M is the intersection of the diagonals, we can express the area in terms of the diagonals AC and BD: [ABCD]=4AC×BD×3
2. **Calculate the area of quadrilateral O1O2O3O4:** - The points O1,O2,O3,O4 are the circumcenters of triangles ABM, BCM, CDM, and DAM, respectively. - The quadrilateral O1O2O3O4 is a parallelogram because the perpendicular bisectors of the sides of the triangles intersect at right angles. - The area of the parallelogram O1O2O3O4 can be calculated using the formula for the area of a parallelogram: Area=base×height - The length of the sides of the parallelogram can be related to the diagonals AC and BD of the original quadrilateral ABCD: O1O2=O3O4=2sin(60∘)BD=3BD O1O4=O2O3=2sin(60∘)AC=3AC - Therefore, the area of O1O2O3O4 is: [O1O2O3O4]=3AC×3BD=3AC×BD
3. Calculate the ratio of the areas: - The ratio of the area of ABCD to the area of O1O2O3O4 is: [O1O2O3O4][ABCD]=3AC×BD4AC×BD×3=3143=43×3=433=23
The final answer is 23
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: NuminaMath-1.5,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.