The Golden Area.

Geometry Level 3

Three 2 × 1 2\times 1 rectangles are arranged as shown. What fraction of the figure is golden?

1 4 \frac{1}{4} 1 7 \frac{1}{7} 1 3 \frac{1}{3} 1 9 \frac{1}{9} 1 8 \frac{1}{8}

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2 solutions

David Vreken
Nov 17, 2019

Label the diagram as follows and draw A D AD and D E DE :

Since B = E = 90 ° \angle B = \angle E = 90° , B D = A E = 2 BD = AE = 2 , and A D = A D AD = AD , A E D D B A \triangle AED \cong \triangle DBA by the hypotenuse-leg congruence theorem.

Since A B = D E = 1 AB = DE = 1 by congruent parts, D C E = A C B \angle DCE = \angle ACB by vertical angles, and B = E = 90 ° \angle B = \angle E = 90° , C E D C B A \triangle CED \cong \triangle CBA by the AAS congruence theorem.

Letting B C = C E = x BC = CE = x , then A C = C D = x 2 + 1 AC = CD = \sqrt{x^2 + 1} by Pythagorean's Theorem, and A E = A C + C E = x 2 + 1 + x = 2 AE = AC + CE = \sqrt{x^2 + 1} + x = 2 which solves to x = 3 4 x = \frac{3}{4} .

Therefore the golden area is A G = 2 1 2 1 3 4 = 3 4 A_G = 2 \cdot \frac{1}{2} \cdot 1 \cdot \frac{3}{4} = \frac{3}{4} , the white area is A W = 3 1 2 A G = 6 3 4 = 21 4 A_W = 3 \cdot 1 \cdot 2 - A_G = 6 - \frac{3}{4} = \frac{21}{4} , and the fraction is A G A W = 3 4 21 4 = 1 7 \frac{A_G}{A_W} = \frac{\frac{3}{4}}{\frac{21}{4}} = \boxed{\frac{1}{7}} .

Thank you for sharing your solution.

Hana Wehbi - 1 year, 6 months ago
Chew-Seong Cheong
Nov 16, 2019

Due to symmetry, the two golden right triangle are congruent. Let the smallest vertex angle of the golden triangle be θ \theta as shown. We note that:

cos θ + 2 sin θ = 2 Using half-angle tangent substitution 1 t 2 1 + t 2 + 4 t 1 + t 2 = 2 and let t = tan θ 2 3 t 2 4 t + 1 = 0 ( 3 t 1 ) ( t 1 ) = 0 t = 1 3 t = 1 is rejected because θ π 2 \begin{aligned} \cos \theta + 2 \sin \theta & = 2 & \small \blue{\text{Using half-angle tangent substitution}} \\ \frac {1-t^2}{1+t^2} + \frac {4t}{1+t^2} & = 2 & \small \blue{\text{and let }t = \tan \frac \theta 2} \\ 3t^2 - 4t + 1 & = 0 \\ (3t-1)(t-1) & = 0 \\ \implies t & = \frac 13 & \small \blue{t = 1 \text{ is rejected because } \theta \ne \frac \pi 2} \end{aligned}

Note that the area of a golden triangle is 1 2 tan θ \dfrac 12 \tan \theta . Therefore, area of the golden region A g = tan θ = 2 t 1 t 2 = 3 4 A_g = \tan \theta = \dfrac {2t}{1-t^2} = \dfrac 34 . The area of the figure is A F = 6 A g = 6 3 4 = 21 4 A_F = 6 - A_g = 6 - \dfrac 34 = \dfrac {21}4 and A g A F = 3 4 × 4 21 = 1 7 \dfrac {A_g}{A_F} = \dfrac 34 \times \dfrac 4{21} = \boxed{\frac 17} .

Thank you Sir.

Hana Wehbi - 1 year, 6 months ago

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