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 angle and the Red angle?
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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 = 1 0 8 ∘ , which is the interior angle of a regular pentagon. Since △ B A D and △ C A F are both isosceles, ∠ B A D = ∠ G A F = 3 6 ∘ , so ∠ C A G = ∠ B A E = 7 2 ∘ . So
∠ B A C = 3 6 0 ∘ − ( ∠ D A G + ∠ B A D + ∠ C A G ) = 3 6 0 ∘ − ( 1 2 0 ∘ + 1 0 8 ∘ ) = 1 3 2 ∘
Since △ B A C is also isosceles,
∠ C B A = ∠ C A B = 2 1 ( 1 8 0 ∘ − ∠ B A C ) = 2 4 ∘
which tells us that
Blue = ∠ C B A + ∠ A B D = 6 0 ∘
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 ∘ and ∠ H I K = ∠ J H M = 3 0 ∘ , then ∠ I H K + ∠ J H N = 1 2 0 ∘ , which also makes up the interior angle of a regular hexagon. Therefore, since
∠ J H I = 3 6 0 ∘ − ( ∠ K H N + 1 2 0 ∘ ) = 1 3 2 ∘ → ∠ J I H = ∠ I J H = 2 4 ∘
It tells us that
Red = ∠ J I H + ∠ H I K = 5 4 ∘
For this problem, Blue > Red
Thanks for posting my image.
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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 2 1 × 5 3 6 0 ∘ = 3 6 ∘ in the case of the regular pentagons and multiples of 2 1 × 6 3 6 0 ∘ = 3 0 ∘ , in the case of regular hexagons.
Moreover, ∠ D A E = 3 6 0 ∘ − ( 1 2 0 ∘ + 2 × 1 0 8 ∘ ) = 2 4 ∘ and ∠ S P T = 3 6 0 ∘ − ( 1 0 8 ∘ + 2 × 1 2 0 ∘ ) = 1 2 ∘
Furthermore, △ A B C and △ P Q R are isoscele triangles, thus ∠ C B A = 2 1 { 1 8 0 ∘ − ( 7 2 ∘ + 2 4 ∘ + 3 6 ∘ ) } = 2 4 ∘ and ∠ R Q P = 2 1 { 1 8 0 ∘ − ( 9 0 ∘ + 1 2 ∘ + 3 0 ∘ ) } = 2 4 ∘
Combining all the above, we find that B l u e a n g l e = 3 6 ∘ + 2 4 ∘ = 6 0 ∘ a n d R e d a n g l e = 3 0 ∘ + 2 4 ∘ = 5 4 ∘ Hence, B l u e > R e d .