Tangled Up

Geometry Level 2

Find the value of the given fraction: ( 45 1 ) tan 1 ° ( 45 3 ) tan 3 1 ° + ( 45 5 ) tan 5 1 ° + ( 45 45 ) tan 45 1 ° 1 ( 45 2 ) tan 2 1 ° + ( 45 4 ) tan 4 1 ° + ( 45 44 ) tan 44 1 ° \frac{ \binom{45}{1}\tan 1°-\binom{45}{3}\tan^3 1°+\binom{45}{5}\tan^5 1°-\dots+\binom{45}{45}\tan^{45} 1°}{1-\binom{45}{2}\tan^2 1°+\binom{45}{4}\tan^4 1°-\dots+\binom{45}{44}\tan^{44} 1°}


The answer is 1.

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1 solution

Chew-Seong Cheong
May 17, 2019

The given quotient can be written as:

Q = n = 0 22 ( 1 ) n ( 45 2 n + 1 ) tan 2 n + 1 1 n = 0 22 ( 1 ) n ( 45 2 n ) tan 2 n 1 = ( n = 0 45 ( 45 n ) i tan n 1 ) ( n = 0 45 ( 45 n ) i tan n 1 ) where i = 1 denotes the imaginary unit, and ( ) = ( ( 1 + i tan 1 ) 45 ) ( ( 1 + i tan 1 ) 45 ) and ( ) , the real and imaginary parts respectively. = ( sec 45 1 e i 4 5 ) ( sec 45 1 e i 4 5 ) By Euler’s formula: e i θ = cos θ + i sin θ = ( e i 4 5 ) ( e i 4 5 ) = ( 1 + i 2 ) ( 1 + i 2 ) = 1 2 1 2 = 1 \begin{aligned} Q & = \frac {\sum_{n=0}^{22} (-1)^n \binom {45}{2n+1}\tan^{2n+1}1^\circ} {\sum_{n=0}^{22} (-1)^n \binom {45}{2n}\tan^{2n}1^\circ} \\ & = \frac {\Im \left(\sum_{n=0}^{45} \binom {45}n i \tan^n 1^\circ \right)} {\Re \left(\sum_{n=0}^{45} \binom {45}n i \tan^n 1^\circ \right)} & \small \color{#3D99F6} \text{where } i = \sqrt{-1} \text{ denotes the imaginary unit, and }\Re(\cdot) \\ & = \frac {\Im \left((1+i \tan 1^\circ)^{45} \right)} {\Re \left((1+i \tan 1^\circ)^{45} \right)} & \small \color{#3D99F6} \text{and } \Im(\cdot) \text{, the real and imaginary parts respectively.} \\ & = \frac {\Im \left(\sec^{45} 1^\circ e^{i45^\circ}\right)}{\Re \left(\sec^{45} 1^\circ e^{i45^\circ}\right)} & \small \color{#3D99F6} \text{By Euler's formula: }e^{i\theta} = \cos \theta + i\sin \theta \\ & = \frac {\Im \left(e^{i45^\circ}\right)}{\Re \left(e^{i45^\circ}\right)} = \frac {\Im \left(\frac {1+i}{\sqrt 2}\right)}{\Re \left(\frac {1+i}{\sqrt 2}\right)} \\ & = \frac {\frac 1{\sqrt 2}}{\frac 1{\sqrt 2}} = \boxed 1 \end{aligned}

Just saying, I don't think this is geometry. More like trigonometry.

Elliott Chen - 2 years ago

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Trigonometry is 2D geometry.

Chew-Seong Cheong - 2 years ago

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