Maths Olympiad Prep

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Geometry Difficulty 7.4 National olympiad, round 2 Find the answer

Convex quadrilateral ABCD ABCD is inscribed in a circle, A\equal60o \angle{A}\equal{}60^o, BC\equalCD\equal1 BC\equal{}CD\equal{}1, rays AB AB and DC DC intersect at point E E, rays BC BC and AD AD intersect each other at point F F. It is given that the perimeters of triangle BCE BCE and triangle CDF CDF are both integers. Find the perimeter of quadrilateral ABCD ABCD.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Given a convex quadrilateral ABCDABCD inscribed in a circle with A=60\angle A = 60^\circ, BC=CD=1BC = CD = 1, and the intersections of rays ABAB and DCDC at point EE, and rays BCBC and ADAD at point FF, we aim to find the perimeter of quadrilateral ABCDABCD given that the perimeters of triangles BCEBCE and CDFCDF are integers.

First, we note that BCD=BAC=60\angle BCD = \angle BAC = 60^\circ since ABCDABCD is cyclic and A=60\angle A = 60^\circ. Let aa and bb be the angles at EE and FF respectively such that a+b=120a + b = 120^\circ.

We consider the triangle CDFCDF. Since CDF=60\angle CDF = 60^\circ, we can use the properties of a 30-60-90 triangle to find that the perimeter of CDF\triangle CDF is an integer. Similarly, the perimeter of BCE\triangle BCE is also an integer.

Using the Law of Sines in CDF\triangle CDF, we have:
1sin(b30)=ysin60    y=sin60sin(b30) \frac{1}{\sin(b - 30^\circ)} = \frac{y}{\sin 60^\circ} \implies y = \frac{\sin 60^\circ}{\sin(b - 30^\circ)}
1sin(b30)=xsin(150b)    x=sin(150b)sin(b30) \frac{1}{\sin(b - 30^\circ)} = \frac{x}{\sin(150^\circ - b)} \implies x = \frac{\sin(150^\circ - b)}{\sin(b - 30^\circ)}

Summing these, we get:
x+y=sin60+sin(150b)sin(b30)=3/2+cosb+3sinb3sinbcosb=3 x + y = \frac{\sin 60^\circ + \sin(150^\circ - b)}{\sin(b - 30^\circ)} = \frac{\sqrt{3}/2 + \cos b + \sqrt{3}\sin b}{\sqrt{3}\sin b - \cos b} = 3

Solving for cosb\cos b and sinb\sin b, we find:
cosb=3314,sinb=1314 \cos b = \frac{3\sqrt{3}}{14}, \quad \sin b = \frac{13}{14}

Using the Law of Sines in ABD\triangle ABD, we have:
ADsina=BDsin60=2    AD=2sina \frac{AD}{\sin a} = \frac{BD}{\sin 60^\circ} = 2 \implies AD = 2\sin a
AB=2sinb=137 AB = 2\sin b = \frac{13}{7}

Since a+b=120a + b = 120^\circ, we have:
sina=sin(120b)=3cosb+sinb2=928+1328=1114 \sin a = \sin(120^\circ - b) = \frac{\sqrt{3}\cos b + \sin b}{2} = \frac{9}{28} + \frac{13}{28} = \frac{11}{14}
AD=2sina=117 AD = 2\sin a = \frac{11}{7}

Thus, the perimeter of quadrilateral ABCDABCD is:
AB+BC+CD+DA=137+1+1+117=387 AB + BC + CD + DA = \frac{13}{7} + 1 + 1 + \frac{11}{7} = \frac{38}{7}

The answer is 387\boxed{\frac{38}{7}}.

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