Infinity square root

Algebra Level 1

2 + 2 + 2 + 2 + = ? \large \sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2 + \cdots}}}} = \, ?

1 2 3 4

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5 solutions

Vinicius Meza
Aug 6, 2015

Make x = 2 + 2 + 2 + . . . x = \sqrt{2+\sqrt{2+\sqrt{2+...}}} Square both sides. Then x 2 = 2 + 2 + 2 + . . . = 2 + x x^{2} = 2 + \sqrt{2+\sqrt{2+...}} = 2+x This results in a quadratic equation x 2 x 2 = 0 x^{2} - x -2 = 0 There are two solutions only, x = 1 x=-1 and x = 2 x=2 But, as x > 0 x > 0 the solution is x = 2 x=2

good job...:)

Mishita Meena - 5 years, 10 months ago

Where is it metioned that x>0

Mayank Prakash - 5 years, 10 months ago

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Hello Mayank. Just note that everything is inside a square root sign. And every number inside is positive. So in the real realm only the positive answer must be considered. Cheers.

Vinicius Meza - 5 years, 10 months ago

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Even iI agree

Pranav Shevkar - 5 years, 10 months ago

Thanks for your answer !

Mayank Prakash - 5 years, 10 months ago

Oh yeah right!

Sumit Gaur - 5 years, 10 months ago

but the answer after computing the square root may be negative.

Swarang Pundlik - 5 years, 7 months ago

Because (something)^1/2 is always positive

Ankita Anshu - 5 years, 7 months ago

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not really... (any real no.)^2 is always positive. the square need not be.

Swarang Pundlik - 5 years, 7 months ago

not exactly always. for example(4)^1/2=-2 or+2

v.s vasu - 5 years, 7 months ago

10+ Great! :)

Никола Ракоњац - 5 years, 7 months ago

Convergence might be an issue. Not in this case, but the first step to take would be to demonstrate that the sequence converges

Omar Monteagudo - 5 years, 7 months ago

How can you substitute root of 2's latr as x.... Whereas the actual x is root 2plus the other roots....lyk actually its n times... But thn it is n-1 no. Of tym's...

Ashish Sethia - 5 years, 10 months ago

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Because it is going upto infinity , infinity-1=infinity

Ankita Anshu - 5 years, 7 months ago
Sai Ram
Aug 12, 2015

Let

x = 2 + 2 + 2 + . . . . . . . . x= \sqrt{2+\sqrt{2+\sqrt{2+........}}}

Now the given expression is equal to x . x.

Now,

x = 2 + x x=\sqrt{2+x} { Since the interior part is also equal to x . x. }

Squaring on both sides,

x 2 = 2 + x . x^2=2+x.

x 2 x 2 = 0. x^2-x-2=0.

x 2 2 x + x 2 = 0. x^2-2x+x-2=0.

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

( x 2 ) ( x + 1 ) . (x-2)(x+1).

Hence the two roots are x = 2 , 1 x=2,-1

Since sum of anything can never be negative , the only answer is 2. 2.

Good solution

Rama Devi - 5 years, 9 months ago

Hey, how to solve same type of question with √132-√132...

Sangeeth Muthukumar - 5 years, 8 months ago

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I didn't understand what you mean. Tell it more clearly. I'll surely help you.

Sai Ram - 5 years, 8 months ago
Ahmed Obaiedallah
Oct 19, 2015

2 + 2 + 2 + 2... = x \sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2...}}}} = x 2 + 2 + 2 + 2... x = x \large\sqrt{2+\underbrace{\sqrt{2+\sqrt{2+\sqrt{2...}}}}_x} = x 2 + x = x \sqrt{2+x} = x 2 + x = x 2 2+x = x^2 x 2 x 2 = 0 x^2-x-2=0 ( x 2 ) ( x + 1 ) = 0 (x-2)(x+1)=0 x = 2 ; x = 1 x=2; x=-1 The answer= 2 \textbf{The answer=}\boxed{\color{#3D99F6}{2}}

Hadia Qadir
Aug 12, 2015

√2+(√2+(√2+(√2...)))=x 2+(√2+(√2+(√2+...)))=x² 2+x=x² x²-x-2=0 (x+1)(x-2) x=(-1), x=2 Positive.. =2 smile emoticon

Use Latex for better display.

Rama Devi - 5 years, 9 months ago

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I'm new here . don't know how to use it

Hadia Qadir - 5 years, 9 months ago

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Click on the formatting guide.

Rama Devi - 5 years, 9 months ago

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@Rama Devi thank you Rama

Hadia Qadir - 5 years, 9 months ago

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@Hadia Qadir Did you learn to write it in ( L a T e X ) \text(LaTeX) ?

Sai Ram - 5 years, 8 months ago

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@Sai Ram not yet . busy in life

Hadia Qadir - 5 years, 8 months ago
Divya Shankhala
Aug 8, 2015

√2+√2+√2+...........=x √2+x=x 2+x=x x 0=x x-x-2 then x=2

Absolutly right divya. Appriciate with you.

Rakesh kumar - 5 years, 10 months ago

Use Latex for better display.

Rama Devi - 5 years, 9 months ago

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