Beauty of Maths

Algebra Level 1

Find the solution to the equation 2 2 2 2... 1 \sqrt{2\sqrt{2\sqrt{2\sqrt{2...}}}} - 1 = x x .


The answer is 1.

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4 solutions

Mohammad Al Ali
May 19, 2014

So here is the solution..

2 2 2 2... 1 = x \sqrt{2\sqrt{2\sqrt{2\sqrt{2...}}}} - 1 = x .

Let y y = 2 2 2... \sqrt{2\sqrt{2\sqrt{2...}}}

Rewriting the equation we obtain,

y 1 y - 1 = x x

Squaring both sides,

( y 1 ) 2 = x 2 (y-1)^{2} = x^{2}

Expanding the brackets of the Left Hand Side we obtain,

y 2 2 y + 1 = x 2 y^{2} - 2{y} + 1 = x^{2}

Calculating for y 2 y^{2} we get that it is ( 2 2 2... ) 2 ( \sqrt{2\sqrt{2\sqrt{2...}}} )^{2}

Or simply,

2 × 2 2 2... 2 \times \sqrt{2\sqrt{2\sqrt{2...}}}

Calculating for 2 y 2y we attain that it is 2 2 2 2... 2\sqrt{2\sqrt{2\sqrt{2...}}}

We notice here that y 2 = 2 y y^{2} = 2y , and so the equation reduces to x 2 1 = 0 x^{2} - 1 = 0

Hence by Difference Of Two Squares, ( x 1 ) ( x + 1 ) = 0 (x-1)(x+1) = 0

However, x = -1 is not a valid solution (extraneous root)

And the solution is: x = 1 \boxed{x=1}

x = 1 x = -1 is not a valid solution to the equation. You introduced extraneous roots when squaring it.

Calvin Lin Staff - 7 years ago

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Indeed it is, was careless of me to check. Checked and fixed, thanks! :)

Mohammad Al Ali - 7 years ago

What is an extraneous root??

Jitesh Mittal - 7 years ago

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It is a number obtained in the process of solving an equation that does not satisfy the equation.

Example of it is:

2 y + 7 + 4 = y \sqrt{2y+7} +4 = y

First, get the radical expression alone on one side of the equation: 2 y + 7 = y 4 \sqrt{2y+7} = y-4

Squaring both sides to get rid of the radical on LHS: 2 y + 7 = ( y 4 ) 2 2y+7 = (y-4)^{2}

Simplify the RHS: 2 y + 7 = y 2 8 y + 16 2y+7 = y^{2}-8y+16

Compute y (put everything on one side): y 2 10 y + 9 = 0 y^{2} - 10y + 9 = 0

Factorising it we obtain: ( y 9 ) ( y 1 ) = 0 (y-9)(y-1) = 0

Hence the solutions for y are , y = 9 , 1 \boxed{ y = 9, 1}

But when plugging y=1 in the equation it wont work: 2 ( 1 ) + 7 + 4 = 1 \sqrt{2(1) + 7} + 4 = 1 is false!

Hence, y=1 is an extraneous root.

Mohammad Al Ali - 7 years ago

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@Mohammad Al Ali I now also don't understand extraneous root

please help me

Divyanshu Joshi - 6 years, 10 months ago

I just solved the radical first(solve a simpler problem) and got a quadratic with roots 0 and 2. Then dealt with subtracting one.

Scott Immel - 7 years ago

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Precisely! However, there is only one solution to this problem which is x = 1 x = 1 , becareful with extraneous roots ;)

Mohammad Al Ali - 7 years ago

very impressive

mezbah uddin - 6 years, 9 months ago
Sunil Pradhan
May 24, 2014

√[2√[2[√2.... = (x + 1) square both sides

2(x + 1) = x² + 2x + 1

2x + 2 = x² + 2x + 1

x² = 1, x = ± 1

@Sunil pradhan -1 is an extraneous root...

rishabh singhal - 7 years ago

Did it just this way(although had eliminated the inadmissable root).But took x-1 in place of x+1.Wont bother if any1 gives slang for such poor mistakes.It befits me.Urgh! Anyway although it seems i am d first person to vote for this solution i prefer it to those above.Looks simple.

Chandrachur Banerjee - 7 years ago
Lu Chee Ket
Aug 14, 2014

2 (x + 1) = (x + 1)^2

=> (x + 1)^2 - 2 (x + 1) = 0

=> (x + 1)(x + 1 - 2) = 0

=> x = 1 as ? - 1 cannot be 0 - 1.

Krishna Garg
Jun 6, 2014

When we take square root of 2 we get 1,41,mutiply by 2 we get2.82,again squareroot we get 1.67 multiply by 2 and srareroot we get finally 4 whose squareroot is2 then take minus 1 so X =1 Ans K.K.GARG,India

Finally we get 4 .. how ? Can u tell me please :(

Ashish Gusain - 5 years, 11 months ago

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