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Algebra Level 3

2 + 2 2 2 2 + 2 2 2 \Large \sqrt{2+ \frac{2}{ \sqrt{2- \frac{2}{ \sqrt{2+ \frac{2}{ \sqrt{2- \frac{2}{ \sqrt{ \ldots }}}}}}}}}

If the value of the infinitely nested expression above equals x x , find the value of 1000 x 1000x .


The answer is 2000.

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2 solutions

Syed Baqir
Sep 16, 2015

x = 2 + 2 2 2 2 + 2 S q u a r i n g B o t h S i d e s x 2 = 2 + 2 2 2 2 + 2 x 2 = 2 + 2 2 2 x ( x 2 2 ) 2 = 4 2 2 x x 4 + 4 4 x 2 = 2 1 1 x ( 1 1 x ) ( x 4 + 4 4 x 2 ) = 2 x 5 x 4 4 x 3 + 4 x 2 + 2 x = 4 W e c a n s e e t h a t i f w e p u t a n y t h i n g i n t h e x i n x 5 x 4 4 x 3 + 4 x 2 w e w i l l g e t 0 H e n c e w e c o u l d s e e t h a t o n l y t e r m l e f t i s 2 x T h e r e f o r e x = 2 a n d 2 ( x ) = 4 x\quad =\quad \sqrt { 2\quad +\quad \frac { 2 }{ \sqrt { 2\quad -\quad \frac { 2 }{ \sqrt { 2\quad +\frac { 2 }{ \sqrt { \dots } } } } } } } \\ \\ Squaring\quad Both\quad Sides\\ \\ { x }^{ 2 }\quad =\quad 2\quad +\quad \frac { 2 }{ \sqrt { 2\quad -\quad \frac { 2 }{ \sqrt { 2\quad +\frac { 2 }{ \sqrt { \dots } } } } } } \\ \Longrightarrow \quad { x }^{ 2 }\quad =\quad 2\quad +\quad \frac { 2 }{ \sqrt { 2-\quad \frac { 2 }{ x } } } \\ \\ \Longrightarrow \quad { (x }^{ 2 }-2)^{ 2 }\quad =\quad \frac { 4 }{ 2\quad -\quad \frac { 2 }{ x } } \\ { x }^{ 4 }+4-4{ x }^{ 2 }\quad =\quad \frac { 2 }{ 1\quad -\quad \frac { 1 }{ x } } \Longrightarrow \quad (1-\frac { 1 }{ x } )({ x }^{ 4 }+4-4{ x }^{ 2 })\quad =\quad 2\\ { x }^{ 5 }-{ x }^{ 4 }-4{ x }^{ 3 }+4{ x }^{ 2 }+2x\quad =4\\ \\ We\quad can\quad see\quad that\quad if\quad we\quad put\quad anything\quad in\quad \\ the\quad x\quad in\quad { x }^{ 5 }-{ x }^{ 4 }-4{ x }^{ 3 }+4{ x }^{ 2 }\quad we\quad will\quad get\quad 0\\ \\ Hence\quad we\quad could\quad see\quad that\quad only\quad term\quad left\quad is\quad 2x\\ \\ Therefore\quad x\quad =\quad 2\quad and\quad 2(x)\quad =\quad 4\\

1000 2 = 2000 \color{#3D99F6} { 1000*2 = \boxed{2000} }

A l t e r n a t i v e \huge \quad \quad \color{#D61F06}{Alternative}

W e m a y a l s o n o t i c e t h a t : x 5 x 4 4 x 3 + 4 x 2 + 2 x = 4 x 5 x 4 4 x 3 + 4 x 2 + 2 x 4 = 0 x 5 x 4 x 3 x 3 + x 2 + x x 3 + x 2 + x x 3 + x 2 + x 2 4 = 0 x 3 ( x 2 x 1 ) x ( x 2 x 1 ) x ( x 2 x 1 ) x 3 + x 2 + x 2 4 = 0 ( x 3 2 x ) ( x 2 x 1 ) x 3 + 2 x 2 4 = 0 ( x 3 2 x ) = x 3 2 x 2 + 4 x 2 x 2 = 0 ( x 2 ) ( x + 1 ) = 0 x = 2 ( O n l y ) We\quad may\quad also\quad notice\quad that:\\ \\ { x }^{ 5 }-{ x }^{ 4 }-4{ x }^{ 3 }+4{ x }^{ 2 }+2x\quad =4\\ \\ { x }^{ 5 }-{ x }^{ 4 }-4{ x }^{ 3 }+4{ x }^{ 2 }+2x-4\quad =\quad 0\\ \\ { x }^{ 5 }-{ x }^{ 4 }-{ x }^{ 3 }\quad \quad -{ x }^{ 3 }+{ x }^{ 2 }+x\quad \quad -{ x }^{ 3 }+{ x }^{ 2 }+x\quad \quad -{ x }^{ 3 }+{ x }^{ 2 }+{ x }^{ 2 }\quad -4\quad =\quad 0\\ \\ { x }^{ 3 }({ x }^{ 2 }-x-1)\quad \quad -x({ x }^{ 2 }-x-1)\quad \quad -x({ x }^{ 2 }-x-1)\quad -{ x }^{ 3 }+{ x }^{ 2 }+{ x }^{ 2 }\quad -4\quad =\quad 0\\ \\ ({ x }^{ 3 }-2x)({ x }^{ 2 }-x-1)\quad \quad -{ x }^{ 3 }+2{ x }^{ 2 }\quad -4\quad =\quad 0\\ \\ \\ ({ x }^{ 3 }-2x)\quad =\quad { x }^{ 3 }-2{ x }^{ 2 }\quad +4\quad \Longrightarrow \quad { x }^{ 2 }-x-2\quad =\quad 0\\ \Longrightarrow (x-2)(x+1)\quad =\quad 0\quad \Longrightarrow \quad x\quad =\quad 2\quad (Only)

A l t e r n a t i v e \huge \color{#20A900}{Alternative}

L e t y b e s i m i l a r e q u a t i o n : y = 2 2 x a n d x = 2 + 2 y x 2 y 2 = 2 x + 2 y ( x + y ) ( x y ) = x + y x y x y = 2 x y x + y 0 ( x y ) x y = 2 ( x y ) = 2 x ( x 2 ) = 2 Let\quad y\quad be\quad similar\quad equation:\\ \\ y\quad =\quad \sqrt { 2\quad -\quad \frac { 2 }{ x } } and\quad x\quad =\quad \sqrt { 2\quad +\quad \frac { 2 }{ y } } \\ { x }^{ 2 }-{ y }^{ 2 }\quad =\quad \frac { 2 }{ x } +\frac { 2 }{ y } \Longrightarrow (x+y)(x-y)=\frac { x+y }{ xy } \Rightarrow x-y=\frac { 2 }{ xy } \\ x+y\quad \ncong \quad 0\quad \\ (x-y)xy\quad =\quad 2\Rightarrow (x-y)\quad =\quad 2\\ x(x-2)=2\\ And Complete further . . . .

x = 2 \boxed {2}

It is absolutely correct, however it can done without forming a fifth degree equation.

Try it by taking two variables.

Akshay Yadav - 5 years, 9 months ago

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Yes I will add that too :D

Syed Baqir - 5 years, 9 months ago

Check my solution now

Syed Baqir - 5 years, 9 months ago

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I think there is a mistake. You have forgotten about negative sign while assuming y y .

Akshay Yadav - 5 years, 9 months ago

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@Akshay Yadav I hope it is correct now :D

Syed Baqir - 5 years, 9 months ago

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@Syed Baqir It is correct my friend.😀

Akshay Yadav - 5 years, 9 months ago

I don't follow the final steps in your first two solutions. Could you explain:

Solution 1) how you know that x^5 - x^4 - 4x^3 + 4x^2 is 0 for all x. For example, (-1)^5 - (-1)^4 - 4(-1)^3 + 4(-1)^2 = 6

Solution 2) I don't understand how the last two lines follow from the line above them

Jordan Cahn - 3 years ago
Chew-Seong Cheong
May 30, 2018

Let y = 2 2 2 + 2 2 2 y = \sqrt{2-\frac 2{\sqrt{2+\frac 2{\sqrt{2-\frac 2{\sqrt \cdots}}}}}} . Then we note that:

{ x = 2 + 2 y x 2 = 2 + 2 y . . . ( 1 ) y = 2 2 x y 2 = 2 2 x . . . ( 2 ) \begin{cases} x = \sqrt{2+\dfrac 2y} & \implies x^2 = 2 + \dfrac 2y & ...(1) \\ y = \sqrt{2-\dfrac 2x} & \implies y^2 = 2 - \dfrac 2x & ...(2) \end{cases}

( 1 ) ( 2 ) : x 2 y 2 = 2 y + 2 x ( x y ) ( x + y ) = 2 ( x + y ) x y x y = 2 x y . . . ( 3 ) \begin{aligned}(1) - (2): \quad x^2 - y^2 & = \frac 2y + \frac 2x \\ (x-y)(x+y) & = \frac {2(x+y)}{xy} \\ \implies x-y & = \frac 2{xy} & ...(3) \end{aligned}

( 1 ) + ( 2 ) : x 2 + y 2 = 4 + 2 y 2 x = 4 + 2 ( x y ) x y ( 3 ) : 2 x y = x y = 4 + ( x y ) 2 x 2 + y 2 = 4 + x 2 2 x y + y 2 x y = 2 x = 2 y \begin{aligned}(1) + (2): \quad x^2 + y^2 & = 4 + \frac 2y - \frac 2x \\ & = 4 + \color{#3D99F6} \frac {2(x-y)}{xy} & \small \color{#3D99F6} (3): \ \frac 2{xy} = x-y \\ & = 4 + \color{#3D99F6} (x-y)^2 \\ x^2 + y^2 & = 4 + x^2-2xy + y^2 \\ \implies xy & = 2 \implies x = \frac 2y \end{aligned}

Therefore,

( 1 ) : x 2 = 2 + 2 y = 2 + x x 2 x 2 = 0 ( x 2 ) ( x + 1 ) = 0 x = 2 x > 0 \begin{aligned} (1): \quad x^2 & = 2 + \frac 2y \\ & = 2 + x \\ \implies x^2 - x - 2 & = 0 \\ (x-2)(x+1) & = 0 \\ \implies x & = 2 & \small \color{#3D99F6} \because x > 0 \end{aligned}

Then 1000 x = 2000 1000x = \boxed{2000} .

Nice solution sir!

Rudrayan Kundu - 2 years, 9 months ago

First prove the convergence of a nested radical.

Guruprasad Ganesh - 1 month, 2 weeks ago

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Yes, you are right.

Chew-Seong Cheong - 1 month, 2 weeks ago

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