Let's Use Complex Things

Geometry Level 4

k = 1 90 sin k k = 1 90 cos k = tan x \large \frac{\sum_{k=1}^{90} \sin k^\circ}{\sum_{k=1}^{90} \cos k^\circ} = - \tan x^\circ

If the equation above holds true for 0 x 180 0 \le x \le 180 , what is the value of x + 1 2 x + \dfrac{1}{2} ?

Note: All are in degrees.


The answer is 135.

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3 solutions

Christian Daang
Aug 3, 2018

Take note that tan 45 = 1 \tan 45 = 1 .

x = 1 90 sin x = 1 + x = 0 89 sin x = 1 + Im ( x = 0 89 ( cis 1 ) x ) = 1 + Im ( ( cis 1 ) 90 ( cis 1 ) 0 ( cis 1 ) 1 ) = 1 + Im ( ( cis 90 ) 1 ( cis 1 ) 1 ) = 1 + Im ( i 1 ( ( cos 1 ) 1 ) + i ( sin 1 ) ( ( cos 1 ) 1 ) i ( sin 1 ) ( ( cos 1 ) 1 ) i ( sin 1 ) ) = 1 + Im ( ( ( sin 1 ) ( cos 1 ) + 1 ) + i ( ( sin 1 ) + ( cos 1 ) 1 ) 2 2 cos 1 ) = 1 + ( 1 2 [ ( 1 cos 1 ) + ( sin 1 ) 1 cos 1 ] ) = 1 + ( 1 2 [ ( 1 cos 1 ) 1 cos 1 + ( sin 1 ) 1 cos 1 ] ) = 1 + ( 1 2 [ 1 + tan ( 179 2 ) ] ) = tan 45 + tan ( 179 2 ) 2 \begin{aligned} \sum_{x = 1}^{90} \sin x &= 1 + \sum_{x = 0}^{89} \sin x \\ &= 1 + \text{Im} \left(\sum_{x = 0}^{89} \left( \text{cis} 1 \right)^x \right) = 1 + \text{Im} \left( \dfrac{ \left( \text{cis} 1 \right)^{90} - \left( \text{cis} 1 \right)^0}{\left( \text{cis} 1 \right) - 1} \right) \\ &= 1 + \text{Im} \left( \dfrac{(\text{cis} 90) - 1}{(\text{cis} 1) - 1} \right) = 1 + \text{Im} \left( \dfrac{i - 1}{\left( (\cos 1) -1\right) + i(\sin1)} \cdot \dfrac{\left( (\cos 1) -1\right) - i(\sin1)}{\left( (\cos 1) -1\right) - i(\sin1)}\right) \\ &= 1 + \text{Im} \left( \dfrac{\left((\sin 1) - (\cos 1) + 1\right) + i\left( (\sin 1) + (\cos 1) - 1\right)}{2 - 2\cos 1}\right) \\ &= 1 + \left( \dfrac{1}{2} \left[ \dfrac{-(1 - \cos 1) + (\sin 1)}{1 - \cos 1}\right]\right) = 1 + \left( \dfrac{1}{2} \left[ \dfrac{-(1 - \cos 1)}{1 - \cos 1} + \dfrac{(\sin 1)}{1 - \cos 1}\right]\right) \\ &= 1 + \left( \dfrac{1}{2} \left[ -1 + \tan \left( \dfrac{179}{2}\right)\right]\right) = \dfrac{\tan 45 + \tan \left( \dfrac{179}{2}\right)}{2} \end{aligned}

and,

x = 1 90 cos x = 1 + x = 0 89 cos x = 1 + Re ( x = 0 89 ( cis 1 ) x ) = 1 + Re ( ( cis 1 ) 90 ( cis 1 ) 0 ( cis 1 ) 1 ) = 1 + Re ( ( cis 90 ) 1 ( cis 1 ) 1 ) = 1 + Re ( i 1 ( ( cos 1 ) 1 ) + i ( sin 1 ) ( ( cos 1 ) 1 ) i ( sin 1 ) ( ( cos 1 ) 1 ) i ( sin 1 ) ) = 1 + Re ( ( ( sin 1 ) ( cos 1 ) + 1 ) + i ( ( sin 1 ) + ( cos 1 ) 1 ) 2 2 cos 1 ) = 1 + ( 1 2 [ ( 1 cos 1 ) + ( sin 1 ) 1 cos 1 ] ) = 1 + ( 1 2 [ ( 1 cos 1 ) 1 cos 1 + ( sin 1 ) 1 cos 1 ] ) = 1 + ( 1 2 [ 1 + tan ( 179 2 ) ] ) = 1 [ ( tan 45 ) ( tan ( 179 2 ) ) ] 2 \begin{aligned} \sum_{x = 1}^{90} \cos x &= -1 + \sum_{x = 0}^{89} \cos x \\ &= -1 + \text{Re} \left(\sum_{x = 0}^{89} \left( \text{cis} 1 \right)^x \right) = -1 + \text{Re} \left( \dfrac{ \left( \text{cis} 1 \right)^{90} - \left( \text{cis} 1 \right)^0}{\left( \text{cis} 1 \right) - 1} \right) \\ &= -1 + \text{Re} \left( \dfrac{(\text{cis} 90) - 1}{(\text{cis} 1) - 1} \right) = -1 + \text{Re} \left( \dfrac{i - 1}{\left( (\cos 1) -1\right) + i(\sin1)} \cdot \dfrac{\left( (\cos 1) -1\right) - i(\sin1)}{\left( (\cos 1) -1\right) - i(\sin1)}\right) \\ &= -1 + \text{Re} \left( \dfrac{\left((\sin 1) - (\cos 1) + 1\right) + i\left( (\sin 1) + (\cos 1) - 1\right)}{2 - 2\cos 1}\right) \\ &= -1 + \left( \dfrac{1}{2} \left[ \dfrac{(1 - \cos 1) + (\sin 1)}{1 - \cos 1}\right]\right) = -1 + \left( \dfrac{1}{2} \left[ \dfrac{(1 - \cos 1)}{1 - \cos 1} + \dfrac{(\sin 1)}{1 - \cos 1}\right]\right) \\ &= -1 + \left( \dfrac{1}{2} \left[ 1 + \tan \left( \dfrac{179}{2}\right)\right]\right) = -\dfrac{1 - \left[ (\tan 45 ) \left( \tan \left( \dfrac{179}{2}\right)\right)\right]}{2} \end{aligned}

Then, the above expression is simply equal to:

tan 45 + tan ( 179 2 ) 1 [ ( tan 45 ) ( tan ( 179 2 ) ) ] = tan ( 179 2 + 45 ) = tan ( 134.5 ) x + 1 2 = 135 - \dfrac{\tan 45 + \tan \left( \dfrac{179}{2}\right)}{1 - \left[ (\tan 45 ) \left( \tan \left( \dfrac{179}{2}\right)\right)\right]} = - \tan \left( \dfrac{179}{2} + 45\right) = - \tan (134.5) \\ \therefore x + \dfrac{1}{2} = \ \boxed{135}

Additional note:

Using the half - tangent formula tan ( x 2 ) = sin x 1 + cos x , \tan \left( \dfrac{x}{2}\right) = \dfrac{\sin x}{1 + \cos x} \ ,

sin 1 1 cos 1 = sin 179 1 + cos 179 = tan ( 179 2 ) . \dfrac{\sin 1}{1 - \cos 1} = \dfrac{\sin 179}{1 + \cos 179} = \tan \left(\dfrac{179}{2} \right) \ .

Albert Yiyi
Aug 6, 2018
  1. insert

  2. product to sum

  3. telescoping sum

  4. sum to prod.

the rest is trivial.

sin ( k A ) cos ( k A ) = 2 sin ( A 2 ) sin ( k A ) 2 sin ( A 2 ) cos ( k A ) = [ cos ( k + 1 2 ) A + cos ( k 1 2 ) A ] [ sin ( k + 1 2 ) A sin ( k 1 2 ) A ] = cos ( n + 1 2 ) A + cos ( 1 1 2 ) A sin ( n + 1 2 ) A sin ( 1 1 2 ) A = 2 sin ( n 2 A ) sin ( n + 1 2 A ) 2 sin ( n 2 A ) cos ( n + 1 2 A ) = tan ( n + 1 2 A ) when A = 1 , n = 90 , tan ( n + 1 2 A ) = tan 45. 5 = tan 134. 5 x + 1 2 = 135 \begin{aligned} \frac{ \sum \sin (kA) }{ \sum \cos (kA) } &= \frac{ \sum {\color{#D61F06}2\sin (\frac{A}{2})} \sin (kA) }{ \sum {\color{#D61F06}2\sin (\frac{A}{2})} \cos (kA) } \\ &= \frac{ \sum \left[ - \cos ({\color{#3D99F6}k} + \frac{1}{2})A + \cos ({\color{#3D99F6}k} - \frac{1}{2})A \right ] }{ \sum \left[ \quad \sin ({\color{#3D99F6}k} + \frac{1}{2})A - \sin ({\color{#3D99F6}k} - \frac{1}{2})A \right ] } \\ &= \frac{ -\cos ({\color{#3D99F6}n} + \frac{1}{2})A + \cos ({\color{#3D99F6}1} - \frac{1}{2})A }{ \quad \sin ({\color{#3D99F6}n} + \frac{1}{2})A - \sin ({\color{#3D99F6}1} - \frac{1}{2})A } \\ &= \frac{ 2\sin (\frac{n}{2}A) \sin (\frac{n+1}{2}A) }{ 2\sin (\frac{n}{2}A) \cos (\frac{n+1}{2}A) } \\ &= \tan (\frac{n+1}{2}A) \\ \text{when } A=1^{\circ} , \ n=90 , \ \tan (\frac{n+1}{2}A) &= \tan 45.5^{\circ} \\ &= -\tan 134.5^{\circ} \\ \therefore x + \frac{1}{2} &= 135 \end{aligned}

Q = k = 1 90 sin k k = 1 90 cos k Since sin θ = cos ( 9 0 θ ) = k = 1 90 cos ( 90 k ) k = 1 90 cos k = k = 0 89 cos k k = 1 89 cos k And cos 9 0 = 0 = cos 0 + k = 1 89 cos k k = 1 89 cos k = 1 k = 1 89 cos k + 1 By Euler’s formula: e θ i = cos θ + i sin θ = 1 k = 1 89 ( e k i ) + 1 where ( ) is the real part of a complex number. = 1 ( k = 1 89 e k i ) + 1 = 1 ( e 1 i e 9 0 i 1 e 1 i ) + 1 = [ ( e 1 i i 1 e 1 i ) ] 1 + 1 = [ ( cos 1 + i sin 1 i 1 cos 1 i sin 1 ) ] 1 + 1 Let t = tan 0. 5 = [ ( 1 t 2 1 + t 2 + 2 t 1 + t 2 i i 1 1 t 2 1 + t 2 2 t 1 + t 2 i ) ] 1 + 1 = [ ( 1 t 2 i ( 1 t ) 2 2 t 2 2 t i ) ] 1 + 1 = [ ( ( 1 t ) ( 1 + t i ( 1 t ) 2 t ( t i ) ) ] 1 + 1 = [ ( ( 1 t ) ( 1 + t i ( 1 t ) ) ( t + i ) 2 t ( t i ) ( t + i ) ) ] 1 + 1 = [ ( ( 1 t ) ( t 2 + 1 + i ( 1 + t 2 ) ) 2 t ( t 2 + 1 ) ) ] 1 + 1 = 2 t 1 t + 1 = 1 + t 1 t = tan 4 5 + tan 0. 5 1 tan 4 5 tan 0. 5 = tan 45. 5 = tan 134. 5 \begin{aligned} Q & = \frac {\sum_{k=1}^{90} \color{#3D99F6} \sin k^\circ}{\sum_{k=1}^{90} \cos k^\circ} & \small \color{#3D99F6} \text{Since }\sin \theta = \cos (90^\circ - \theta) \\ & = \frac {\sum_{k=1}^{90} \color{#3D99F6} \cos (90-k)^\circ}{\sum_{k=1}^{90} \cos k^\circ} \\ & = \frac {\sum_{\color{#D61F06}k=0}^{\color{#D61F06}89} \cos k^\circ}{\sum_{k=1}^{\color{#3D99F6}89} \cos k^\circ} & \small \color{#3D99F6} \text{And } \cos 90^\circ = 0 \\ & = \frac {\cos 0^\circ + \sum_{k=1}^{89} \cos k^\circ}{\sum_{k=1}^{89} \cos k^\circ} \\ & = \frac 1{\sum_{k=1}^{89} \color{#3D99F6} \cos k^\circ} + 1 & \small \color{#3D99F6} \text{By Euler's formula: }e^{\theta i} = \cos \theta + i\sin \theta \\ & = \frac 1{\sum_{k=1}^{89} \color{#3D99F6}\Re \left(e^{k^\circ i} \right)} + 1 & \small \color{#3D99F6} \text{where } \Re (\cdot) \text{ is the real part of a complex number.} \\ & = \frac 1{\Re \left(\sum_{k=1}^{89} e^{k^\circ i} \right)} + 1 \\ & = \frac 1{\Re \left(\frac {e^{1^\circ i}-e^{90^\circ i}}{1-e^{1^\circ i}} \right)} + 1 \\ & = \left[\Re \left(\frac {e^{1^\circ i}-i}{1-e^{1^\circ i}} \right)\right]^{-1} + 1 \\ & = \left[\Re \left(\frac {\cos 1^\circ + i\sin 1^\circ -i}{1-\cos 1^\circ - i\sin 1^\circ} \right)\right]^{-1} + 1 & \small \color{#3D99F6} \text{Let }t = \tan 0.5^\circ \\ & = \left[\Re \left(\frac {\frac {1-t^2}{1+t^2} + \frac {2t}{1+t^2}i -i}{1- \frac {1-t^2}{1+t^2} - \frac {2t}{1+t^2}i} \right)\right]^{-1} + 1 \\ & = \left[\Re \left(\frac {1-t^2-i(1-t)^2}{2t^2 - 2ti} \right)\right]^{-1} + 1 \\ & = \left[\Re \left(\frac {(1-t)(1+t-i(1-t)}{2t(t- i)} \right)\right]^{-1} + 1 \\ & = \left[\Re \left(\frac {(1-t)(1+t-i(1-t))\color{#3D99F6}(t+i)}{2t(t- i)\color{#3D99F6}(t+i)} \right)\right]^{-1} + 1 \\ & = \left[\Re \left(\frac {(1-t)(t^2+1+i(1+t^2))}{2t(t^2+1)} \right)\right]^{-1} + 1 \\ & = \frac {2t}{1-t} + 1 = \frac {1+t}{1-t} = \frac {\tan 45^\circ + \tan 0.5^\circ}{1-\tan 45^\circ \tan 0.5^\circ} \\ & = \tan 45.5^\circ = - \tan 134.5^\circ \end{aligned}

Since Q = tan x Q = - \tan x , x = 134.5 \implies x = 134.5 and x + 1 2 = 135 x+\frac 12 = \boxed{135} .

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