An Irregular Pentagram

Geometry Level 3

An irregular pentagram that can be inscribed in a unit circle has four congruent angle tips and a fifth angle tip that is twice as big as one of the others.

If the area of its inner pentagon is p q \frac{\sqrt{p}}{q} , where p p and q q are square-free integers, find p + q p + q .


The answer is 9.

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2 solutions

Mark Hennings
Nov 27, 2019

The pentagram has four vertex angles of 3 0 30^\circ and one vertex angle of 6 0 60^\circ . Its inner pentagon is made up of four congruent right-angled triangles with perpendicular sides of lengths 1 2 \tfrac12 and 1 2 3 \tfrac{1}{2\sqrt{3}} , and hence the pentagon has area 4 × 1 2 × 1 2 × 1 2 3 = 3 6 4 \times \tfrac12 \times \tfrac12 \times \tfrac{1}{2\sqrt{3}} \; = \; \frac{\sqrt{3}}{6} making the answer 3 + 6 = 9 3+6 = \boxed{9} .

David Vreken
Nov 30, 2019

Since the sum of two sets of exterior angles of the interior pentagon is 2 360 ° = 720 ° 2 \cdot 360° = 720° , and the sum of the angles of the five triangles around it is 5 180 ° = 900 ° 5 \cdot 180° = 900° , the angle tips have a sum of 900 ° 720 ° = 180 ° 900° - 720° = 180° .

Letting x x be the measurement of one of the four congruent angle tips, the fifth angle tip is 2 x 2x , which means 4 x + 2 x = 180 ° 4x + 2x = 180° , so that x = 30 ° x = 30° . Therefore, the five angle tips are 30 ° 30° , 30 ° 30° , 30 ° 30° , 30 ° 30° , and 60 ° 60° .

Let the pentagram be labelled as follows:

By symmetry, since D A C = 60 ° \angle DAC = 60° , then F A C = 30 ° \angle FAC = 30° . By the inscribed angle theorem, since A D B = 30 ° \angle ADB = 30° , then A F B = 60 ° \angle AFB = 60° . As radii, A F = B F = 1 AF = BF = 1 . Solving A F J \triangle AFJ , A J F = 90 ° \angle AJF = 90° , F J = 1 2 FJ = \frac{1}{2} , and A J = 3 2 AJ = \frac{\sqrt{3}}{2} , and solving B K F \triangle BKF , B K F = 90 ° \angle BKF = 90° and K F = 1 2 KF = \frac{1}{2} , and solving A K I \triangle AKI , A K = A F K F = 1 2 AK = AF - KF = \frac{1}{2} and K I = 3 6 KI = \frac{\sqrt{3}}{6} .

The area of A F J \triangle AFJ is then A A F J = 1 2 1 2 3 2 = 3 8 A_{\triangle AFJ} = \frac{1}{2} \cdot \frac{1}{2} \cdot \frac{\sqrt{3}}{2} = \frac{\sqrt{3}}{8} , and the area of A K I \triangle AKI is then A A K I = 1 2 3 6 1 2 = 3 24 A_{\triangle AKI} = \frac{1}{2} \cdot \frac{\sqrt{3}}{6} \cdot \frac{1}{2} = \frac{\sqrt{3}}{24} .

By symmetry, the area of the inner pentagon is A F G H I J = 2 ( A A F J A A K I ) = 2 ( 3 8 3 24 ) = 3 6 A_{FGHIJ} = 2 (A_{\triangle AFJ} - A_{\triangle AKI}) = 2(\frac{\sqrt{3}}{8} - \frac{\sqrt{3}}{24}) = \frac{\sqrt{3}}{6} .

Therefore, p = 3 p = 3 , q = 6 q = 6 , and p + q = 9 p + q = \boxed{9} .

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