In the following figure A E = A D = D C = A C = F G = 1
What is the length of C F ?
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That explains why it was so hard to construct with straightedge and compass.
By Menelaus' Theorem considering the transversal A D G of the triangle C E F , A E C A × D F E D × G C F G = − 1 (here lengths are signed). Thus 1 × C F 2 − 1 3 × C F + 1 1 = 1 and hence C F 2 − 1 ( C F + 1 ) = 3 , and so C F is a positive real root of the equation ( x 2 − 1 ) ( x + 1 ) 2 − 3 x 4 + 2 x 3 − 2 x − 4 ( x + 2 ) ( x 3 − 2 ) = 0 = 0 = 0 and so C F = 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 = 1 2 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 3 0 = 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 = 1 8 0 − 1 2 0 = 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 = . 7 6 6 . ⟹ C F = 1 2 + . 7 6 6 2 = 1 . 2 5 9 6 6 .
Letting C F = x , and using the Law of Sines, we solve for x
x 2 − 1 S i n ( A r c C s c ( x ) − 6 π ) = 1 S i n ( 6 π )
which delivers x = 1 . 2 5 9 9 2
Considering that ∠ C D F = 2 π , sin ∠ D F C = C F C D = x 1
Let D G = y . Employing the sine law, we see that for triangle D F G :
1 sin 6 π = y sin ∠ D F G
2 1 = y sin ( π − ∠ D F C
2 1 = y sin ∠ D F C
2 1 = y x 1
2 1 = x y 1
x y = 2
Employing the cosine law, we see that for triangle C D G :
( x + 1 ) 2 = 1 2 + y 2 − 2 ( 1 ) ( y ) cos 3 2 π
x 2 + 2 x + 1 = 1 + y 2 − 2 y ( − 2 1 )
x 2 + 2 x = y 2 + y
x ( x + 2 ) = y ( y + 1 )
x ( x + x y ) = y ( y + 1 )
x x ( 1 + y ) = y ( y + 1 )
x 2 = y
x x 2 = x y
x 1 + 2 = 2
x 3 = 2
x = 3 2
With some angle chasing we can find out that ∠ B F G = 3 0 ° and ∠ C D G = 1 2 0 ° Let D F = y and C F = x
Observing that ∠ C D F = 9 0 ° we know from pytaghorean theorem that
1 2 + y 2 = x 2 and x > y > 0
Now, from law of sines in △ F D G
sin ( ∠ F G D ) y = sin ( 3 0 ° ) 1 ∴ y = 2 × sin ( ∠ F G D )
Then, law of sines in △ C D G
sin ( ∠ F G D ) 1 = sin ( 1 2 0 ° ) 1 + x ∴ x = 2 1 3 csc ( ∠ F G D ) − 1
putting al together and solving those equations we get the answer 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.
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We first note that triangle A E D is isosceles with vertex angle 1 2 0 ∘ , so ∠ E D A = 3 0 ∘ , which then makes ∠ F D G = 3 0 ∘ and ∠ C D F = 9 0 ∘ .
Since triangle C D F is right, we know that sin θ = x 1 . Also, applying Sine Law to triangle D F G , we get
1 sin 3 0 ∘ 2 1 2 1 y = y sin α = y sin θ [ Because θ = 1 8 0 ∘ − α ] = x y 1 [ Using sin θ = x 1 ] = x 2
Now we use Cosine Law on triangle C D G :
( x + 1 ) 2 x 2 + 2 x + 1 x 4 + 2 x 3 + x 2 x 4 + 2 x 3 x 3 ( x + 2 ) x 3 x = 1 2 + y 2 − 2 ( 1 ) ( y ) cos 1 2 0 ∘ = 1 + x 2 4 + x 2 [ Now multiply by x 2 ] = x 2 + 2 x + 4 = 2 x + 4 = 2 ( x + 2 ) [ Since x cannot be - 2 , we can divide by ( x + 2 ) ] = 2 = 3 2 ≈ 1 . 2 5 9