Golden Ratio in 5-gon and 6-gon
The following has been posted by Tran Quang Hung at the CutTheKnotMath facebook page, with a proof by Leo Giugiuc.
Indeed, due to symmetry, suffice it to prove that $\displaystyle\frac{DR}{ER}=\varphi.\;$
Choose, WLOG, $A=0,\;$ $F=2,\;$ $E=3+i\sqrt{3},\;$ and $D=2+2i\sqrt{3}.\;$ We have,$\displaystyle\frac{A-F}{P-F}=\cos\frac{3\pi}{5}+i\sin\frac{3\pi}{5},\;$ implying
$\displaystyle\begin{align} P&=2-2\cos\frac{3\pi}{5}+2i\sin\frac{3\pi}{5}\\ &=2+2\sin\frac{2\pi}{5}+2i\sin\frac{3\pi}{5}\\ &=4\cos^2\frac{\pi}{5}+2i\sin\frac{3\pi}{5}\\ &=\varphi^2+2i\sin\frac{3\pi}{5}\\ &=1+\varphi+2i\sin\frac{3\pi}{5}. \end{align}$
From here, the line $PR\;$ is $x=1+\varphi,\;$ so that $R=1+\varphi+ki\;$ (we are not interested in the value of k.) It follows that $\displaystyle\overrightarrow{DR}=\varphi-1+mi=\frac{1}{\varphi}+mi\;$ and $\displaystyle\overrightarrow{RE}=2-\varphi+ni=\frac{1}{\varphi^2}+ni\;$ and, finally, $\displaystyle\frac{DR}{ER}=\frac{\displaystyle\frac{1}{\varphi}}{\displaystyle\frac{1}{\varphi^2}}=\varphi.$

Golden Ratio
- Golden Ratio in Geometry
- Golden Ratio in Regular Pentagon
- Golden Ratio in an Irregular Pentagon
- Golden Ratio in a Irregular Pentagon II
- Inflection Points of Fourth Degree Polynomials
- Wythoff's Nim
- Inscribing a regular pentagon in a circle - and proving it
- Cosine of 36 degrees
- Continued Fractions
- Golden Window
- Golden Ratio and the Egyptian Triangle
- Golden Ratio by Compass Only
- Golden Ratio with a Rusty Compass
- From Equilateral Triangle and Square to Golden Ratio
- Golden Ratio and Midpoints
- Golden Section in Two Equilateral Triangles
- Golden Section in Two Equilateral Triangles, II
- Golden Ratio is Irrational
- Triangles with Sides in Geometric Progression
- Golden Ratio in Hexagon
- Golden Ratio in Equilateral Triangles
- Golden Ratio in Square
- Golden Ratio via van Obel's Theorem
- Golden Ratio in Circle - in Droves
- From 3 to Golden Ratio in Semicircle
- Another Golden Ratio in Semicircle
- Golden Ratio in Two Squares
- Golden Ratio in Two Equilateral Triangles
- Golden Ratio As a Mathematical Morsel
- Golden Ratio in Inscribed Equilateral Triangles
- Golden Ratio in a Rhombus
- Golden Ratio in Five Steps
- Between a Cross and a Square
- Four Golden Circles
- Golden Ratio in Mixtilinear Circles
- Golden Ratio With Two Equal Circles And a Line
- Golden Ratio in a Chain of Polygons, So to Speak
- Golden Ratio With Two Unequal Circles And a Line
- Golden Ratio In a 3x3 Square
- Golden Ratio In a 3x3 Square II
- Golden Ratio In Three Tangent Circles
- Golden Ratio In Right Isosceles Triangle
- Golden Ratio Poster
- Golden Ratio Next to the Poster
- Golden Ratio In Rectangles
- Golden Ratio In a 2x2 Square: Without And Within
- Golden Ratio With Two Unequal Circles And a Line II
- Golden Ratio in Equilateral and Right Isosceles Triangles
- Golden Ratio in a Butterfly Astride an Equilateral Triangle
- The Golden Pentacross
- 5-Step Construction of the Golden Ratio, One of Many
- Golden Ratio in 5-gon and 6-gon
- Golden Ratio in an Isosceles Trapezoid with a 60 degrees Angle
- Golden Ratio in Pentagon And Two Squares
- Golden Ratio in Pentagon And Three Triangles
- Golden Ratio in a Mutually Beneficial Relationship
- Star, Six Pentagons and Golden Ratio
- Rotating Square in Search of the Golden Ratio
- Cultivating Regular Pentagons
- Golden Ratio in an Isosceles Trapezoid with a 60 degrees Angle II
- More of Gloden Ratio in Equilateral Triangles
- Golden Ratio in Three Regular Pentagons
- Golden Ratio in Three Regular Pentagons II
- Golden Ratio in Wu Xing
- Golden Ratio In Three Circles And Common Secant
- Flat Probabilities on a Sphere
- Golden Ratio in Square And Circles
- Golden Ratio in Square
- Golden Ratio in Two Squares, Or, Perhaps in Three
- Golden Ratio in Isosceles Triangle
- Golden Ratio in Circles
- Golden Ratio in Isosceles Triangle II
- Golden Ratio in Yin-Yang

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