Two angle bisector

Geometry Level 2


The answer is 13.4164.

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2 solutions

Chew-Seong Cheong
Jun 17, 2020

Let A C = x AC=x and C = θ \angle C = \theta . Then we have:

B C : x cos θ = 8 2 cos θ 2 A B : x sin θ = 9 cos ( 4 5 θ 2 ) = 9 2 ( cos θ 2 + sin θ 2 ) A B B C : tan θ = 9 ( cos θ 2 + sin θ 2 ) 16 cos θ 2 = 9 16 ( 1 + tan θ 2 ) Let t = tan θ 2 32 t 1 t 2 = 9 ( 1 + t ) 32 t = 9 ( 1 + t t 2 t 3 ) 9 t 3 + 9 t 2 + 23 t 9 = 0 t = tan θ 2 = 1 3 cos θ 2 = 3 10 B C : x 4 5 = 8 2 3 10 cos θ = 2 ( 3 10 ) 2 1 = 4 5 x = 3 20 13.4 \begin{aligned} BC: \quad x\cos \theta & = 8\sqrt 2 \cos \frac \theta 2 \\ AB: \quad x \sin \theta & = 9 \cos \left(45^\circ - \frac \theta 2\right) \\ & = \frac 9{\sqrt 2}\left(\cos \frac \theta 2 + \sin \frac \theta 2 \right) \\ \frac {AB}{BC}: \quad \tan \theta & = \frac {9\left(\cos \frac \theta 2 + \sin \frac \theta 2 \right)}{16 \cos \frac \theta 2} \\ & = \frac 9{16} \left(1 + \blue{\tan \frac \theta 2} \right) & \small \blue{\text{Let }t = \tan \frac \theta 2} \\ \frac {32t}{1-t^2} & = 9(1+t) \\ 32t & = 9(1+t-t^2 - t^3) \\ 9t^3 + 9t^2+23t-9 & = 0 \\ \implies t & = \tan \frac \theta 2 = \frac 13 & \small \blue{\implies \cos \frac \theta 2 = \frac 3{\sqrt{10}}} \\ BC: \quad x \cdot \frac 45 & = 8\sqrt 2 \cdot \frac 3{\sqrt{10}} & \small \blue{\implies \cos \theta = 2 \left(\frac 3{\sqrt{10}}\right)^2 - 1 = \frac 45} \\ \implies x & = 3\sqrt{20} \approx \boxed{13.4} \end{aligned}

We can write C = 90 ° A , h cos A = 9 cos A 2 , h sin A = 8 2 cos ( 45 ° A 2 ) C=90\degree-A,h\cos A=9\cos \dfrac {A}{2}, h\sin A=8\sqrt 2\cos \left (45\degree -\dfrac {A}{2}\right ) , where h = A C h=|\overline {AC}|

8 ( cos A 2 + sin A 2 ) = h sin A \implies 8\left (\cos \dfrac{A}{2}+\sin \dfrac {A}{2}\right ) =h\sin A

h 2 = 64 ( 1 + sin A ) sin 2 A = 81 ( 1 + cos A ) 2 cos 2 A \implies h^2=\dfrac {64(1+\sin A) }{\sin^2 A}=\dfrac {81(1+\cos A) }{2\cos^2 A}

sin A = 81 + 81 cos A 209 cos 2 A 81 cos 3 A 128 cos 2 A \implies \sin A=\dfrac {81+81\cos A-209\cos^2 A-81\cos^3 A}{128\cos^2 A}

Squaring, converting into cosine function only and solving we get

cos A = 3 5 , sin A = 4 5 , h 2 = 180 \cos A=\dfrac 35,\sin A=\dfrac 45,h^2=180 .

So, h = A C = 180 13.4164 h=|\overline {AC}|=\sqrt {180}\approx \boxed {13.4164} .

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