A beautiful construction

Geometry Level 5

In the following figure A E = A D = D C = A C = F G = 1 AE=AD=DC=AC=FG=1

What is the length of C F CF ?


The answer is 1.25992.

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7 solutions

Zico Quintina
Jun 29, 2018

We first note that triangle A E D AED is isosceles with vertex angle 12 0 120^{\circ} , so E D A = 3 0 \angle EDA = 30^{\circ} , which then makes F D G = 3 0 \angle FDG = 30^{\circ} and C D F = 9 0 \angle CDF = 90^{\circ} .

Since triangle C D F CDF is right, we know that sin θ = 1 x \sin \theta = \dfrac{1}{x} . Also, applying Sine Law to triangle D F G DFG , we get

sin 3 0 1 = sin α y 1 2 = sin θ y [ Because θ = 18 0 α ] 1 2 = 1 x y [ Using sin θ = 1 x ] y = 2 x \begin{aligned} \dfrac{\sin 30^{\circ}}{1} &= \dfrac{\sin \alpha}{y} \\ \\ \dfrac{1}{2} &= \dfrac{\sin \theta}{y} \qquad \small \text{[ Because } \theta = 180^{\circ} - \alpha \ ] \\ \\ \dfrac{1}{2} &= \dfrac{1}{x y} \qquad \small \text{[ Using } \sin \theta = \dfrac{1}{x} \ ] \\ \\ y &= \dfrac{2}{x} \end{aligned}

Now we use Cosine Law on triangle C D G CDG :

( x + 1 ) 2 = 1 2 + y 2 2 ( 1 ) ( y ) cos 12 0 x 2 + 2 x + 1 = 1 + 4 x 2 + 2 x [ Now multiply by x 2 ] x 4 + 2 x 3 + x 2 = x 2 + 2 x + 4 x 4 + 2 x 3 = 2 x + 4 x 3 ( x + 2 ) = 2 ( x + 2 ) [ Since x cannot be - 2 , we can divide by ( x + 2 ) ] x 3 = 2 x = 2 3 1.259 \begin{aligned} (x + 1)^2 &= 1^2 + y^2 - \ 2 \ (1) (y) \cos 120^{\circ} \\ \\ x^2 + 2x + 1 &= 1 + \dfrac{4}{x^2} + \dfrac{2}{x} \qquad \small \text{[ Now multiply by } x^2 \ ] \\ \\ x^4 + 2x^3 + x^2 &= x^2 + 2x + 4 \\ \\ x^4 + 2x^3 &= 2x + 4 \\ \\ x^3 (x + 2) &= 2 (x + 2) \qquad \small \text{[ Since } x \text{ cannot be -} 2 \text{, we can divide by } (x + 2) \ ] \\ \\ x^3 &= 2 \\ \\ x &= \sqrt[3]{2} \approx \boxed{1.259} \end{aligned}

That explains why it was so hard to construct with straightedge and compass.

Jeremy Galvagni - 2 years, 10 months ago
Mark Hennings
Jul 25, 2018

By Menelaus' Theorem considering the transversal A D G ADG of the triangle C E F CEF , C A A E × E D D F × F G G C = 1 \frac{CA}{AE} \times \frac{ED}{DF} \times \frac{FG}{GC} \; = \; -1 (here lengths are signed). Thus 1 × 3 C F 2 1 × 1 C F + 1 = 1 1 \times \frac{\sqrt{3}}{\sqrt{CF^2-1}} \times \frac{1}{CF+1} \; = \; 1 and hence C F 2 1 ( C F + 1 ) = 3 \sqrt{CF^2-1}(CF+1) = \sqrt{3} , and so C F CF is a positive real root of the equation ( x 2 1 ) ( x + 1 ) 2 3 = 0 x 4 + 2 x 3 2 x 4 = 0 ( x + 2 ) ( x 3 2 ) = 0 \begin{aligned} (x^2-1)(x+1)^2 - 3 & = \; 0 \\ x^4 + 2x^3 - 2x - 4 & = \; 0 \\ (x+2)(x^3 - 2) & = \; 0 \end{aligned} and so C F = 2 3 CF= \boxed{\sqrt[3]{2}} .

L e t D F = X . F r o m t h e g i v e d a t a , Δ C A D i s 1 1 1 , s C A D = A D C = 6 0 o . S o D A E = 12 0 o , b u t A E = A D , i n Δ A E D , F D G = E D A = 3 0 o , a n d C D F = 9 0 o . r t . Δ C D F , F C D = a t a n X . U s i n g S i n e L a w i n Δ F D G , 1 / s i n 30 = X / s i n D G F , D G F = a s i n ( X / 2 ) . I n Δ C D G , s D G F + F C D = 180 120 = 6 0 o S o l v i n g a s i n ( X / 2 ) + a t a n X = 6 0 o b y t r i a l a n d e r r o r , w e g e t X = . 766. C F = 1 2 + . 76 6 2 = 1.25966. Let~DF=X.\\ ~~~~\\ From~the~give~data,\\ \Delta~CAD~is~~1-1-1, ~\therefore~~\angle s~~ CAD=ADC=60^o.\\ So~\angle~DAE=120^o, ~~but~AE=AD,\\ \therefore~in~\Delta~AED,~~\angle~FDG=EDA=30^o,~~and~~\angle~CDF=90^o.\\ ~~~~\\ rt.~\angle~\Delta~CDF,~~\angle~FCD=atanX.\\ ~~~~\\ Using~Sine~Law~in~\Delta~FDG, ~1/sin30=X/sinDGF,\\ \therefore~\angle~DGF=asin(X/2).\\ ~~~~\\ In~\Delta~CDG,~~\angle s ~DGF+FCD=180-120=60^o\\ Solving~asin(X/2)+atanX=60^o~ by~ trial~ and ~error,\\ we~get~X=.766.\\ ~~~~\\ \implies~CF=\sqrt{1^2+.766^2}=\huge \color{#D61F06}{1.25966}.

Michael Mendrin
Jul 26, 2018

Letting C F = x CF=x , and using the Law of Sines, we solve for x x

S i n ( A r c C s c ( x ) π 6 ) x 2 1 = S i n ( π 6 ) 1 \dfrac { Sin( ArcCsc(x) -\frac{\pi}{6} ) } { \sqrt{x^2-1} } = \dfrac { Sin(\frac{\pi}{6}) } {1}

which delivers x = 1.25992 x=1.25992

Ajit Athle
Jul 25, 2018

See the diagram.

Noel Lo
Jul 23, 2018

Considering that C D F = π 2 \angle CDF=\dfrac{\pi}{2} , sin D F C = C D C F = 1 x \sin \angle DFC=\dfrac{CD}{CF}=\dfrac{1}{x}

Let D G = y DG=y . Employing the sine law, we see that for triangle D F G DFG :

sin π 6 1 = sin D F G y \dfrac{\sin\dfrac{\pi}{6}}{1}=\dfrac{\sin \angle DFG}{y}

1 2 = sin ( π D F C y \dfrac{1}{2}=\dfrac{\sin (\pi-\angle DFC}{y}

1 2 = sin D F C y \dfrac{1}{2}=\dfrac{\sin \angle DFC}{y}

1 2 = 1 x y \dfrac{1}{2}=\dfrac{\dfrac{1}{x}}{y}

1 2 = 1 x y \dfrac{1}{2}=\dfrac{1}{xy}

x y = 2 xy=2

Employing the cosine law, we see that for triangle C D G CDG :

( x + 1 ) 2 = 1 2 + y 2 2 ( 1 ) ( y ) cos 2 π 3 (x+1)^2=1^2+y^2-2(1)(y) \cos \dfrac{2\pi}{3}

x 2 + 2 x + 1 = 1 + y 2 2 y ( 1 2 ) x^2+2x+1=1+y^2-2y\left(-\dfrac{1}{2}\right)

x 2 + 2 x = y 2 + y x^2+2x=y^2+y

x ( x + 2 ) = y ( y + 1 ) x(x+2)=y(y+1)

x ( x + x y ) = y ( y + 1 ) x(x+xy)=y(y+1)

x x ( 1 + y ) = y ( y + 1 ) xx\cancel{(1+y)}=y\cancel{(y+1)}

x 2 = y x^2=y

x x 2 = x y xx^2=xy

x 1 + 2 = 2 x^{1+2}=2

x 3 = 2 x^3=2

x = 2 3 x=\sqrt[3]{2}

Relue Tamref
Jun 29, 2018

With some angle chasing we can find out that B F G = 30 ° \angle BFG = 30 ° and C D G = 120 ° \angle CDG = 120 ° Let D F = y DF = y and C F = x CF = x

Observing that C D F = 90 ° \angle CDF = 90 ° we know from pytaghorean theorem that

1 2 + y 2 = x 2 1^2 + y^2 = x^2 and x > y > 0 x>y>0

Now, from law of sines in F D G \triangle FDG

y sin ( F G D ) = 1 sin ( 30 ° ) \frac{y}{\sin(\angle FGD)} = \frac{1} {\sin(30°)} \therefore y = 2 × sin ( F G D ) y = 2 \times \sin(\angle FGD)

Then, law of sines in C D G \triangle CDG

1 sin ( F G D ) = 1 + x sin ( 120 ° ) x = 1 2 3 csc ( F G D ) 1 \frac{1}{\sin(\angle FGD)} = \frac{1+x}{\sin(120° )} \therefore x = \frac{1}{2} \sqrt{3} \csc(\angle FGD) - 1

putting al together and solving those equations we get the answer 2 3 \sqrt[3]{2}

It looks like you have a point B hiding under point D which makes it hard to read. I wonder if you can fix this? It would also be nice if your picture indicated which sides are equal to 1.

Jeremy Galvagni - 2 years, 10 months ago

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