Pent-hexagon & Hex-pentagon

Geometry Level 3

Both diagrams illustrate the combinations of three unit regular polygons. On the left, the hexagon is adjacent to two pentagons touching at a vertex, whereas on the right the pentagon is adjacent to two hexagons also touching at a vertex. The angles are formed as shown above.

Which of the following must be true about the values of the Blue \color{#3D99F6}\text{Blue} angle and the Red \color{#D61F06}\text{Red} angle?

Blue = Red {\color{#3D99F6}\text{Blue}} = {\color{#D61F06}\text{Red}} Blue > Red {\color{#3D99F6}\text{Blue}} > {\color{#D61F06}\text{Red}} Blue < Red {\color{#3D99F6}\text{Blue}} < {\color{#D61F06}\text{Red}}

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2 solutions

Label some of the polygon vertices as seen in the figure.

All angles which are inscribed in the circumcircles of the regular polygons have measures which are multiples of 1 2 × 360 5 = 36 \dfrac{1}{2}\times \dfrac{360{}^\circ }{5}=36{}^\circ in the case of the regular pentagons and multiples of 1 2 × 360 6 = 30 \dfrac{1}{2}\times \dfrac{360{}^\circ }{6}=30{}^\circ , in the case of regular hexagons.

Moreover, D A E = 360 ( 120 + 2 × 108 ) = 24 \angle DAE=360{}^\circ -\left( 120{}^\circ +2\times 108{}^\circ \right)=24{}^\circ and S P T = 360 ( 108 + 2 × 120 ) = 12 \angle SPT=360{}^\circ -\left( 108{}^\circ +2\times 120{}^\circ \right)=12{}^\circ

Furthermore, A B C \triangle ABC and P Q R \triangle PQR are isoscele triangles, thus C B A = 1 2 { 180 ( 72 + 24 + 36 ) } = 24 \angle CBA=\frac{1}{2}\left\{ 180{}^\circ -\left( 72{}^\circ +24{}^\circ +36{}^\circ \right) \right\}=24{}^\circ and R Q P = 1 2 { 180 ( 90 + 12 + 30 ) } = 24 \angle RQP=\dfrac{1}{2}\left\{ 180{}^\circ -\left( 90{}^\circ +12{}^\circ +30{}^\circ \right) \right\}=24{}^\circ

Combining all the above, we find that B l u e a n g l e = 36 + 24 = 60 a n d R e d a n g l e = 30 + 24 = 54 {\color{#3D99F6}{Blue}} \ angle=36{}^\circ +24{}^\circ =60{}^\circ \ \ \ \ \ and \ \ \ \ {\color{#D61F06}{Red}} \ angle=30{}^\circ +24{}^\circ =54{}^\circ Hence, B l u e > R e d \boxed{{\color{#3D99F6}{Blue}} > {\color{#D61F06}{Red}}} .

Michael Huang
Mar 3, 2021

The following is the image by Chew Seong Cheong


For the solution, define the points as follows:

Note that B A D + B A E = F A C + C A G = 10 8 \angle BAD + \angle BAE = \angle FAC + \angle CAG = 108^{\circ} , which is the interior angle of a regular pentagon. Since B A D \bigtriangleup BAD and C A F \bigtriangleup CAF are both isosceles, B A D = G A F = 3 6 \angle BAD = \angle GAF = 36^{\circ} , so C A G = B A E = 7 2 \angle CAG = \angle BAE = 72^{\circ} . So

B A C = 36 0 ( D A G + B A D + C A G ) = 36 0 ( 12 0 + 108 ) = 13 2 \angle BAC = 360^{\circ} - (\angle DAG + \angle BAD + \angle CAG) = 360^{\circ} - (120^{\circ} + 108\circ) = 132^{\circ}

Since B A C \bigtriangleup BAC is also isosceles,

C B A = C A B = 1 2 ( 18 0 B A C ) = 2 4 \angle CBA = \angle CAB = \dfrac{1}{2}\left(180^{\circ} - \angle BAC\right) = 24^{\circ}

which tells us that

Blue = C B A + A B D = 6 0 {\color{#3D99F6}\text{Blue}} = \angle CBA + \angle ABD = 60^{\circ}

We can follow the similar steps for the pentagon tangent to two hexagons. Angle-chasing, we see that since L H J = J H N = 9 0 \angle LHJ = \angle JHN = 90^{\circ} and H I K = J H M = 3 0 \angle HIK = \angle JHM = 30^{\circ} , then I H K + J H N = 12 0 \angle IHK + \angle JHN = 120^{\circ} , which also makes up the interior angle of a regular hexagon. Therefore, since

J H I = 36 0 ( K H N + 12 0 ) = 13 2 J I H = I J H = 2 4 \angle JHI = 360^{\circ} - (\angle KHN + 120^{\circ}) = 132^{\circ} \rightarrow \angle JIH = \angle IJH = 24^{\circ}

It tells us that

Red = J I H + H I K = 5 4 {\color{#D61F06}\text{Red}} = \angle JIH + \angle HIK = 54^{\circ}

For this problem, Blue > Red \boxed{{\color{#3D99F6}\text{Blue}} > {\color{#D61F06}\text{Red}}}

Thanks for posting my image.

Chew-Seong Cheong - 3 months, 1 week ago

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