Equilateral triangle ABC has circumcircle Ω. Points D and E are chosen on minor arcs AB and AC of Ω respectively such that BC=DE. Given that triangle ABE has area 3 and triangle ACD has area 4, find the area of triangle ABC.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
A rotation by 120∘ about the center of the circle will take ABE to BCD, so BCD has area 3. Let AD=x,BD=y, and observe that ∠ADC=∠CDB=60∘. By Ptolemy's Theorem, CD=x+y. We have 4=[ACD]=21AD⋅CD⋅sin60∘=43x(x+y)3=[BCD]=21BD⋅CD⋅sin60∘=43y(x+y) By dividing these equations find x:y=4:3. Let x=4t,y=3t. Substitute this into the first equation to get 1=43⋅7t2. By the Law of Cosines, AB2=x2+xy+y2=37t2 The area of ABC is then 4AB23=737
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: Omni-MATH,
licensed Apache-2.0.
Statement reproduced verbatim; metadata (topic, difficulty) added by this project.