Three 2 × 1 rectangles are arranged as shown. What fraction of the figure is golden?
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Due to symmetry, the two golden right triangle are congruent. Let the smallest vertex angle of the golden triangle be θ as shown. We note that:
cos θ + 2 sin θ 1 + t 2 1 − t 2 + 1 + t 2 4 t 3 t 2 − 4 t + 1 ( 3 t − 1 ) ( t − 1 ) ⟹ t = 2 = 2 = 0 = 0 = 3 1 Using half-angle tangent substitution and let t = tan 2 θ t = 1 is rejected because θ = 2 π
Note that the area of a golden triangle is 2 1 tan θ . Therefore, area of the golden region A g = tan θ = 1 − t 2 2 t = 4 3 . The area of the figure is A F = 6 − A g = 6 − 4 3 = 4 2 1 and A F A g = 4 3 × 2 1 4 = 7 1 .
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Label the diagram as follows and draw A D and D E :
Since ∠ B = ∠ E = 9 0 ° , B D = A E = 2 , and A D = A D , △ A E D ≅ △ D B A by the hypotenuse-leg congruence theorem.
Since A B = D E = 1 by congruent parts, ∠ D C E = ∠ A C B by vertical angles, and ∠ B = ∠ E = 9 0 ° , △ C E D ≅ △ C B A by the AAS congruence theorem.
Letting B C = C E = x , then A C = C D = x 2 + 1 by Pythagorean's Theorem, and A E = A C + C E = x 2 + 1 + x = 2 which solves to x = 4 3 .
Therefore the golden area is A G = 2 ⋅ 2 1 ⋅ 1 ⋅ 4 3 = 4 3 , the white area is A W = 3 ⋅ 1 ⋅ 2 − A G = 6 − 4 3 = 4 2 1 , and the fraction is A W A G = 4 2 1 4 3 = 7 1 .