Another Golden Ratio in Regular Pentagon
Problem
Solution 1
WLOG, $OP=1.\;$ Recollect that $\displaystyle\varphi=2\cos\frac{\pi}{5}.\;$
In $\Delta OEM,\;$ $\displaystyle\frac{1}{OE}=\cos\frac{\pi}{5},\;$ so that $OE=\displaystyle\frac{2}{\varphi},\;$ implying $EP=\displaystyle\frac{2-\varphi}{\varphi}.\;$ Hence, too, $MR=\displaystyle\frac{2-\varphi}{\varphi}.\;$ From which
$\displaystyle NR=\frac{1}{\varphi}-\frac{2-\varphi}{\varphi}=\frac{\varphi-1}{\varphi}=\frac{1}{\varphi^2}.\;$
Triangles $QRN\;$ and $PQO\;$ are similar, such that
It follows that, $\displaystyle \frac{QR}{PR}=\varphi^2-1=\varphi.$
Solution 2
As above, $DO=\displaystyle\frac{2}{\varphi},\;$ $\displaystyle NO=\frac{1}{\varphi},\;$ $\displaystyle NQ=1-\frac{1}{\varphi}=\frac{\varphi-1}{\varphi}=\frac{1}{\varphi^2},\;$ $\displaystyle\frac{PR}{QR}=\frac{NO}{NQ}=\frac{\displaystyle\frac{1}{\varphi}}{\displaystyle\frac{1}{\varphi^2}}=\varphi.$
Acknowledgment
The problem has been posted on the CutTheKnotMath facebook page by Tran Quang Hung and commented on by Leo Giugiuc with Solution 1.
The new construction is a byproduct of the beautifully austere Tran Quang Hung's observation: $\displaystyle\frac{OF}{FG}=\varphi,\;$ where $F\;$ is the midpoint of $BO:$
Golden Ratio
- Golden Ratio in Geometry
- Golden Ratio in Regular Pentagon
- Another 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
|Contact| |front page| |Contents| |Geometry|
Copyright © 1996-2018 Alexander Bogomolny71925163