GeometryDifficulty 7.4National olympiad, round 2Find the answer
Convex quadrilateral ABCD is inscribed in a circle, ∠A\equal60o, BC\equalCD\equal1, rays AB and DC intersect at point E, rays BC and AD intersect each other at point F. It is given that the perimeters of triangle BCE and triangle CDF are both integers. Find the perimeter of quadrilateral ABCD.
A number or a short expression. Spacing and $ signs are ignored.
Solution
Given a convex quadrilateral ABCD inscribed in a circle with ∠A=60∘, BC=CD=1, and the intersections of rays AB and DC at point E, and rays BC and AD at point F, we aim to find the perimeter of quadrilateral ABCD given that the perimeters of triangles BCE and CDF are integers.
First, we note that ∠BCD=∠BAC=60∘ since ABCD is cyclic and ∠A=60∘. Let a and b be the angles at E and F respectively such that a+b=120∘.
We consider the triangle CDF. Since ∠CDF=60∘, we can use the properties of a 30-60-90 triangle to find that the perimeter of △CDF is an integer. Similarly, the perimeter of △BCE is also an integer.
Using the Law of Sines in △CDF, we have: sin(b−30∘)1=sin60∘y⟹y=sin(b−30∘)sin60∘ sin(b−30∘)1=sin(150∘−b)x⟹x=sin(b−30∘)sin(150∘−b)
Summing these, we get: x+y=sin(b−30∘)sin60∘+sin(150∘−b)=3sinb−cosb3/2+cosb+3sinb=3
Solving for cosb and sinb, we find: cosb=1433,sinb=1413
Using the Law of Sines in △ABD, we have: sinaAD=sin60∘BD=2⟹AD=2sina AB=2sinb=713
Since a+b=120∘, we have: sina=sin(120∘−b)=23cosb+sinb=289+2813=1411 AD=2sina=711
Thus, the perimeter of quadrilateral ABCD is: AB+BC+CD+DA=713+1+1+711=738
The answer is 738.
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