Functions here and there

Geometry Level 5

Solve for x x ,

sin [ 2 cos 1 { cot ( 2 tan 1 x ) } ] = 0 \sin\left[2\cos^{-1}\left\{\cot\left(2\tan^{-1}x\right)\right\}\right]=0

If x x has n n solutions, all of which can be represented in surd form, x i = a i + b i x_i = a_i+\sqrt{b_i}

Evaluate { n + i = 1 n ( a i + b i ) } \large \text{Evaluate }\left\{n + \sum\limits_{i=1}^n \left({\left|a_i\right| + \left|\sqrt{b_i}\right|}\right)\right\}


The answer is 17.656854249492380195206754896839.

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2 solutions

S i n [ 2 C o s 1 { C o t ( 2 T a n 1 X ) } ] = 0 , 2 C o s 1 { C o t ( 2 T a n 1 X ) } = 0 , o r 2 π O R π . . . . . . . . . . . . . . 0 2 C o s 1 { C o t ( 2 T a n 1 X ) } 2 π . C o s 1 { C o t ( 2 T a n 1 X ) } = 0 , o r π O R π 2 . C o t ( 2 T a n 1 X ) = 1 , o r 1 O R 0 . L e t θ = 2 T a n 1 X , X = T a n θ 2 . T a n θ = 2 X 1 X 2 . 1 X 2 2 X = C o t θ = C o t ( 2 T a n 1 X ) . 1 X 2 2 X = ± 1 O R 0. S o l v i n g t h e q u a d r a t i c , n = 6 , X 1 = 1 + 2 , X 2 = 1 2 . X 3 = 1 + 2 , X 4 = 1 2 . X 5 = 1 , X 6 = 1. { n + i = 1 n ( a i + b i ) } = 6 + 2 1 + 2 ± 2 + 2 1 + 2 ± 2 + 2 ± 1 = 17.657 Sin[2*Cos^{-1}\{Cot ( 2*Tan^{-1}X)\}]=0,\\ \implies ~2*Cos^{-1}\{Cot ( 2*Tan^{-1}X)\}=0,~~or~~2*\pi~~~OR~~~\pi.\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ............. 0\le {\Large \color{#3D99F6}{2} }*Cos^{-1}\{Cot ( 2*Tan^{-1}X)\} \le 2*\pi.\\ \implies ~Cos^{-1}\{Cot ( 2*Tan^{-1}X)\}=0,~~or~~\pi~~~OR~~~\dfrac{\pi}2.\\ \implies~ \color{#3D99F6}{ Cot ( 2*Tan^{-1}X)=1,~~or~~-1~~~OR~~~0}.\\ Let~~ \theta=2*Tan^{-1}X, ~~\implies X=Tan\dfrac \theta 2. \\ \therefore~Tan \theta=\dfrac {2X}{1 - X^2}. \\ \implies~ \color{#3D99F6}{ \dfrac {1 - X^2} {2X}=Cot \theta=Cot ( 2*Tan^{-1}X).}\\ \therefore~ \dfrac {1 - X^2} {2X}=\pm 1~~~OR~~~~~~~0.\\ Solving~~ the~~ quadratic,\\ n=6,\\ X_1=-1 +\sqrt2,~~~~~~~X_2= -1 - \sqrt2.\\ X_3= 1 +\sqrt2,~~~~~~~~~~X_4= 1 - \sqrt2 .\\ X_5=1 ,~~~~~~~~~~~~~~~~~~~~ X_6=- 1.\\ \left\{n + \sum\limits_{i=1}^n \left({\left|a_i\right| + \left|\sqrt{b_i}\right|}\right)\right\}\\ =6+2*|- 1|+ 2*|\pm \sqrt2|+2*|1|+ 2*|\pm \sqrt2|+2*|\pm 1|\\ = \Large \color{#D61F06}{ 17.657}

sin ( e x p ) = 0 \sin(exp)=0 does not only imply e x p = 0 exp = 0 .

Kishore S. Shenoy - 5 years, 8 months ago

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You are correct. I am adding to my solution. Please correct the answer.

Niranjan Khanderia - 5 years, 8 months ago

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What can I do for you? I did not quite understand...

Kishore S. Shenoy - 5 years, 8 months ago

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@Kishore S. Shenoy My solution and answer I have corrected. So the old answer must be corrected. As the author I think you can correct the answer. Is my solution correct ?

Niranjan Khanderia - 5 years, 8 months ago

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@Niranjan Khanderia I can't correct my answer and I don't think your answer is correct. I see a slight mistake in it...

Kishore S. Shenoy - 5 years, 8 months ago

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@Kishore S. Shenoy Yes. I have made a mistake again !! I missed the first 2 inside. You are correct. I am correcting my mistake. Thank you.

Niranjan Khanderia - 5 years, 8 months ago
Kishore S. Shenoy
Oct 16, 2015

Let's take the expression, sin [ 2 cos 1 { cot ( 2 tan 1 x ) } ] = 0 \sin\left[2\cos^{-1}\left\{\cot\left(2\tan^{-1}x\right)\right\}\right]=0

For cos 1 x \cos^{-1} x Principle domain is θ [ 0 , π ] 2 θ [ 0 , 2 π ] \theta \in\left[0, \pi\right]\\\Rightarrow 2\theta \in [0, 2\pi]

So, θ = 0 , π 2 , π \theta = 0, \dfrac{\pi}2, \pi . Applying cos \cos , cot { 2 tan 1 x } = 1 , 0 , 1 \cot\left\{2\tan^{-1}x\right\} = 1, 0, -1

OR 1 x 2 2 x = 1 , 0 , 1 \dfrac{1-x^2}{2x}=1,0,-1

Solving, n = 6 x 1 , x 2 = 1 ± 2 x 3 , x 4 = ± 1 x 5 , x 6 = 1 ± 2 \Huge\boxed{ n=6\\\begin{aligned}x_1,~x_2&=-1\pm\sqrt 2\\x_3,~ x_4 &= \pm 1\\x_5,~x_6 &= 1 \pm\sqrt 2 \end{aligned}}

Moderator note:

What makes this problem interesting?

@Calvin Lin What else that these key points such as Bijective domain and all...LOL!!

Kishore S. Shenoy - 5 years, 7 months ago

@Calvin Lin Sir, could you please answer the Moderator's question.....

Aaghaz Mahajan - 2 years, 10 months ago

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