Impossibly Long

Algebra Level 2

x + x + x + = 5 , x = ? \large \sqrt{x+\sqrt{x+\sqrt{x +\ldots }}} = 5, \quad\quad\quad x = \ ?


The answer is 20.

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55 solutions

Ben Frankel
Dec 27, 2013

Squaring both sides we get,

x + x + x + = 25 x + \sqrt{x+\sqrt{x+\sqrt{\dots}}} = 25

We know from the original equation that x + x + x + = 5 \sqrt{x+\sqrt{x+\sqrt{x+\dots}}} = 5 , so we substitute.

x + 5 = 25 x = 20 x + 5 = 25 \Rightarrow x = \boxed{20}

the rule is that if root x +root x + root x=5, and the radicals are nested and it's an infinite series, then 5=root(x+5). so, you can derive the same one step equation in the solution. Really amazing solution by the way. What level math are you learning right now?

Isaac Jacobs - 7 years, 5 months ago

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cool

Abdullah khan - 7 years, 4 months ago

That is a really nice solution

John M. - 7 years, 5 months ago

bhai log sidha concept jasiey ocean sey ek drop water nikalna . same concept for this and for other q of this type rootx + rootx + rootx + ........=5 using concept agey sey chod key ek x baki ko dekhen to =5 to root under jo tha uska value root under x+5 =5 ab root shifting and now x+5=25 x=20;

Ankit Verma - 7 years, 4 months ago

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bhaiya root shifting kya hota hai....uske bare me bataiye pls ....

AnWesa Royce - 7 years ago

Got the Same

Mehul Arora - 6 years, 7 months ago

Simple solution!

Ng Donn - 7 years, 5 months ago

Thanks! Ihave new trick to share now :)

El Filibusterismo - 7 years, 5 months ago

a good one

B Sharan - 7 years, 5 months ago

Sorry Ben I lied to you, it's not Landau. It's just some common rule.

Isaac Jacobs - 7 years, 5 months ago

Good explanation! \sqrt{x+sqrt(x)...}=5

Joe Lingle - 7 years, 5 months ago

I'm still no really understand !

Lightning JJ - 7 years, 5 months ago

good

Dania Alvi - 7 years, 5 months ago

Nice

Shi Qin Yew - 7 years, 4 months ago

Nice solution!

LAWRENE FRANCIS - 7 years, 4 months ago

nice thinking

Sowmy Vivek - 7 years, 4 months ago

nice good thought

Janu Janu - 7 years, 4 months ago

superb

Harsha Reddy - 7 years, 4 months ago

keep it up, good job

kiran digal - 7 years, 4 months ago

Cognitive query, nice solution

uk Ansari - 7 years, 4 months ago

nice and easy

Kavyansh Chourasia - 7 years, 4 months ago

woooh! i really love math

Julius Celio - 7 years, 4 months ago

nice

Vijay Patil - 7 years, 4 months ago

amazing solution

Aneeq Ali - 7 years, 4 months ago

OR simply solve it using the calculator, the answer will be approaching 20

Khaled Mohamed - 7 years, 4 months ago

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  1. You can't assume that the nested radical converges to an integer... yes it gets you the right answer on brilliant, but it's nowhere near a proof.
  2. Is it not simpler to solve it algebraically and get the exact answer without need for a calculator at all?

Ben Frankel - 7 years, 4 months ago

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I agree, but as an engineer student, we are taught to find the quickest and simplest solution to any problem which is by using the calculator

Khaled Mohamed - 7 years, 4 months ago

généralement, si x étant un nombre réel supérieur ou égal à 1 :
{\sqrt {x+{\sqrt {x+{\sqrt {x+{\sqrt {x+\cdots }}}}}}}}={\frac {1+{\sqrt {(4\,x+1)}}}{2}} Alors : {\frac {1+{\sqrt {(4\,x+1)}}}{2}}=5
x=80/4=20 .

Simo Fert - 7 years, 4 months ago

Very easy........................

Rajat Nair - 7 years, 3 months ago

good

Veeranthagan Raj Veera - 7 years, 2 months ago

awesome solution.

TIRTHANKAR GHOSH - 7 years, 2 months ago

Let LHS expression=a. Squaring both sides yield

x + a = 25 -> x + 5 = 25 -> x = 20

Joselito Lloren - 7 years, 1 month ago

the best solution !!

Ahmed Salah - 6 years, 11 months ago

But this will only give us approx value

Omkar Chavan - 5 years, 7 months ago

My goodness, I am getting slow, thanks for this. Need to practice my math skills to achieve my goal on being a math teacher.

Axl Dela Cruz - 7 years, 5 months ago

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Good luck :)

Ben Frankel - 7 years, 5 months ago

thanks for reminding me i am slow...

Satyam Mishra - 7 years, 5 months ago

We should get this to 20 upvotes

Devery Sheridan - 7 years, 5 months ago

how i can write sqrt(x) symbolically please help.

mukesh kumar - 7 years, 5 months ago

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You use LaTeX. In order to start LaTeX, you write \ ( without a space in between, and then \ ) to end it. The way to make a square root sign in LaTeX is \sqrt{...}

Ben Frankel - 7 years, 5 months ago

Do you know what Landau's algorithm is?

Isaac Jacobs - 7 years, 5 months ago

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Isn't Landau's algorithm used for denesting finite radicals of radicals? This is an infinite radical, and so I think that that doesn't apply here, though I don't know much about Landau's algorithm.

Ben Frankel - 7 years, 5 months ago

If someone could please highlight X and say, "Here it is," that would make me happy

Isaac Jacobs - 7 years, 5 months ago

nice try

Kirk Hammet - 7 years, 4 months ago

good one

Tasnim Rawat - 7 years, 3 months ago

what level is this question

micah mzumara - 7 years, 2 months ago

clever approach, i solved it using guess and check

Sandesh PauDell - 7 years, 5 months ago
Prasun Biswas
Dec 28, 2013

x + x + x + . . . . = 5 \sqrt{x+\sqrt{x+\sqrt{x+....}}}=5 .....(i)

On squaring both sides, we get,

x + x + x + x + . . . . = 25 x+\sqrt{x+\sqrt{x+\sqrt{x+....}}}=25

x + 5 = 25 x+5=25 [From (i)]

x = 20 x=\boxed{20}

good

naidu prase - 7 years, 4 months ago

Did you use Landau's Theorem?

Isaac Jacobs - 7 years, 5 months ago

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No, I just squared both sides of the original equation and then substituted the value of x + x + x + . . . \sqrt{x+\sqrt{x+\sqrt{x+...}}} with the value from original equation to get the value of x = 20 x=\boxed{20}

Prasun Biswas - 7 years, 5 months ago

i solved this q in my syllabus book

Sajjad Hyder - 7 years, 4 months ago

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Hello! you have told that you have solved this query in your syllabus book.I am a medical student and have no expertise in the field of mathematics. For the preparation of aptitude test will you kindly tell me that book and as well some others(in case if you know), so that I will be very humble to you.

uk Ansari - 7 years, 4 months ago

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reply must I am in dire need of suggestions as test is looming ahead me

uk Ansari - 7 years, 4 months ago
Biswajit Basak
Dec 27, 2013

We can consider a sequence as a 1 = x , a 2 = x + x , a 3 = x + x + x , . . . . a_{1} = \sqrt{x} , a_{2} = \sqrt{x+\sqrt{x}} , a_{3} = \sqrt{x+\sqrt{x+\sqrt{x}}} , .... Clearly for any n > 1 , a n + 1 = x + a n n>1 , a_{n+1} = \sqrt{x+a_{n}} that implies , a n + 1 2 = x + a n a^{2}_{n+1} = x + a_{n} We are given that lim n a n = 5 \lim_{n\rightarrow\infty} a_{n} = 5 . So we take the both hand limits and get, x = lim n a n + 1 2 lim n a n = 5 2 5 = 20 x = \lim_{n\rightarrow\infty} a^{2}_{n+1} - \lim_{n\rightarrow\infty} a_{n} = 5^{2} - 5 = 20

Thanks for writing this answer, I really want to understand how the question was solved this way. Would you mind answer a few difficulties that I have in understanding this answer?

1) for n > 1, a(n + 1) = sqrt(x + an) - I just don't understand this,

2) Limit of a(n) = 5, a(n+1) = 5^2? I just get how! And in the end, why did you subtract the limit, was there any logical connection of sorts?

Sorry, but sometimes I really am slow in understanding Maths!

Aaditya Geed - 7 years, 5 months ago

nice one~~

Tony Lan - 7 years, 5 months ago

different thinking..

Praveen India - 7 years, 5 months ago

very nice

Sheran Abbasi - 7 years, 4 months ago

thank you :D you made me understand ^_^

Lovely Javier - 7 years, 4 months ago

Why are you trying to eat around your head, still you can directly eat

Pramesh S - 7 years, 4 months ago

i think its very easy with limits

Ahmad Eldeeb - 7 years, 4 months ago

Great solution. So elegant. Harder math applied simply becomes truly elegant and intelligible any solution.

Carlos E. C. do Nascimento - 7 years, 2 months ago
Caroline Sudipa
Dec 28, 2013

Let $$(\sqrt ( x+\sqrt (x+ \sqrt (x+... \infty)=a ....(i) $$ Again $$(\sqrt ( x+\sqrt (x+ \sqrt (x+... \infty)=5 ....(ii) $$ Hence, $$a=5$$ Squaring on both sides of (ii): $$x+ (\sqrt ( x+\sqrt (x+ \sqrt (x+... \infty)=25$$ $$\Rightarrow x+a=25$$ $$\Rightarrow x=25-5$$ $$\Rightarrow x=20$$

Good xplanation.,thanks

Star Buck - 7 years, 5 months ago

\sqrt{x}

nitin gupta - 7 years, 5 months ago

nice

Atul Kaushik - 7 years, 5 months ago

Great explanation.

Manas Uniyal - 7 years, 5 months ago

great

samugam kandasamy - 7 years, 4 months ago
Oliver Welsh
Dec 27, 2013

We have: \begin{align} \sqrt{x + \sqrt{x + \sqrt{x + \dots}}} &= 5 \\ x + \sqrt{x + \sqrt{x + \dots}} &= 25 \\ x + 5 &= 25 \\ x &= \fbox{20} \end{align}

Tristan Shin
Dec 27, 2013

x + x + x + . . . = 5 \sqrt{x+\sqrt{x+\sqrt{x+...}}} = 5

x + 5 = 5 \sqrt{x+5} = 5

x + 5 = 25 x+5=25

x = 20 x=\boxed{20}

Ajay Maity
Dec 27, 2013

Given, x + x + x + . . . = 5 \sqrt{x + \sqrt{x + \sqrt{x + ...}}} = 5 .......... (i)

Here, there are infinite x x terms inside the square roots.

Square both sides,

x + x + x + x + . . . = 25 x + \sqrt{x + \sqrt{x + \sqrt{x + ...}}} = 25 .......... (ii)

Here, there are one less than infinite x x 's, which is same as infinite x x terms in the square roots.

Therefore, using (i), we can write (ii) as

x + 5 = 25 x + 5 = 25

x = 20 x = 20

That's the answer!

Lai Xu
Dec 30, 2013

Squaring on both sides to the equation then we get x + x + x + = 25 x+\sqrt{x+\sqrt{x+\dots}}=25 Then substituting x + x + x + = 5 \sqrt{x+\sqrt{x+\sqrt{x+\dots}}}=5 into the equation above, we will get x + 5 = 25 x+5=25 . Finally we get x = 20 x=\boxed{20}

√x+√x+√++........=√x+5=5(because it is an infinite series) squaring on both sides, x+5=25, x=20

Jessica Yung
Dec 27, 2013

Square both sides and subtract x from both sides:

x + x + x + . . . = 25 x+\sqrt{x+\sqrt{x+...}} = 25

x + x + . . . = 25 x \sqrt{x+\sqrt{x+...}} = 25 -x

Then substitute in the original equation, and you have

5 = 25 x 5 = 25-x

x = 20 x=\boxed{20} .

Ratnottam Das
Dec 27, 2013

x + x + x + . . . . . . = 5 \sqrt{x+\sqrt{x+\sqrt{x+......}}}=5 or x + 5 = 5 \sqrt{x+5}=5 or x + 5 = 25 x+5=25 or x = 20 x=\boxed{20}

If I take the geometric series approach, then the powers of x are 1/2, 1/4, 1/8, and the series sums to 1.

Then, x = 5

This is different from the "other / correct" answer.

Why does the geometric series approach give a different answer, and which one is correct?

Star Light - 7 years, 5 months ago

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Ok. I got it. I cant apply a geometric series here becuase of the plus signs.

Star Light - 7 years, 5 months ago
Henry Okafor
Jan 19, 2014

Let x + . . . . . \sqrt{x + .....} = a. Then, a = 5

Therefore, x + a \sqrt{x + a} = 5

Thus, x + a = 25

But a =5

Therefore, x = 20

nice method!

Rupsa Mukherjee - 7 years, 4 months ago
Ashutosh Tripathi
Jan 30, 2016

The question is very easy,even though it looks tough n + n + n + = 5 \large \sqrt{n+\sqrt{n+\sqrt{n+\dots}}} = 5 And as the concept for such question is to use the formula x = 1 + 4 n + 1 2 \large x = \frac{1+\sqrt{4n+1}}{2} Hence we do... 1 + 4 n + 1 2 = 5 \large \frac{1+\sqrt{4n+1}}{2} = 5 We multiply RHS and LHS by 2 1 + 4 n + 1 = 10 \large 1+\sqrt{4n+1} = 10 Then we subtract 1 from RHS and LHS 4 n + 1 = 9 \large \sqrt{4n+1} = 9 Squaring both the sides 4 n + 1 = 81 \large 4n+1 = 81 Subtracting 1 from both sides 4 n = 80 \large 4n = 80 Dividing both sides with 4 n = 20 \large n = 20

Aravind M
Oct 15, 2014

That 8 legged cat made my little brother laugh out of his wits....! Awesome question and the apt (octapede... :-) )image'-.....!

I had no idea what I was doing so I first converted it to X 1 / 2 + X 1 / 4 + X 1 / 8 . . . = 5 X^{1/2}+X^{1/4}+X^{1/8}...=5

Then I realized that X + X 1 / 2 + X 1 / 4 + X 1 / 8 . . . = 5 2 X+X^{1/2}+X^{1/4}+X^{1/8}...=5^{2}

and that X + X 1 / 2 + X 1 / 4 + X 1 / 8 . . . = 5 + X X+X^{1/2}+X^{1/4}+X^{1/8}...=5+X

So I just did 25 = X + 5 25=X+5

Brijbihari Shukla
Mar 11, 2014

Let y = √ x + √ x + √ x + √ x +……………….. Now according the question y = 5 y^2 = 25 = x + √ x + √ x + √ x +……………….. or we can say y^2 = y + x 25 = 5 + x or x = 20

Krishna Kumar
Feb 15, 2014

root inside root go to infinite times then we assume , if we hold a first root x and until go to 5 because this turns TO INFINITE. let's simplfies sqrt(x+5)=5; x+5=25; x=20;

Martin Raj Kumar
Feb 5, 2014

\sqrt{x}+\sqrt{x}+\sqrt{x}..........=5 \sqrt{x}+5 =5 Squaring bothsides, x+5=23 Therefore,x=20

Rohan K
Feb 1, 2014

In the equation, keep the roots other than the value of the first 'x' as 5. Solving, root of (x+5)=5 Squaring, we get, x+5=25.So,x=20.

√(x +√(x + √(x + . . . . .))) = 5 ---------(i) Squaring both sides, we get x +√(x + √(x + . . . . .)) = 25 or, x + 5 = 25 [from (i)] So, x = 25 – 5 = 20

Pramesh S
Jan 20, 2014

square on both sides you will get

x + sqrt(x(sqrt(xsqrt(x)))..= 25 , Now the value of sqrt(x(sqrt(xsqrt(x))) = 5 from given question. So replace it with 5.

you will get x + 5 = 25 , which implies x = 20

Solution: √(x+√(x+√(x+⋯)))=5 ≫√(x+5)=5 [ as √(x+√(x+√(x+⋯)))=5 ] ≫x+5=25 ∴x=20

Square the expression on both the sides. One x will come out. Replace the remaining x's with 5 as they continue till infinite. You will have x + 5 = 25 x+5=25 . Hence x = 20 x=20 .

Govindarasu Mv
Jan 18, 2014

x=20

Shakir Khatti
Jan 17, 2014

√(x+√(x+√(x+⋯)) ) =5 〖(√(x+√(x+√(x+⋯)) ) )〗^2=5^2 x+√(x+√(x+⋯)) =25 As we know that √(x+√(x+√(x+⋯)) ) =5 x+5=25 x=25-5 x=20

Sandeep Sharma
Jan 16, 2014

20

y = sqrt(x + sqrt(x + sqrt(x + ….)))

y2 = x + sqrt(x + sqrt(x + sqrt(x + ….)))

y2 = x + y

Now, we have y = 5

52 = x + 5

25 = x + 5

x = 20

Pushpesh Kumar
Jan 14, 2014

GIVEN THAT,

√(x+√(x+√(x+⋯)=5

SO WE CAN WRITE THAT,

√(x+5)=5

NOW,

x+5=5^2

x+5=25

THEREFORE, x=25-5=20

Saqib Munir
Jan 10, 2014

squaring both sides, then put \sqrt{x}+\sqrt{x}+....+5 then it will give us x=20 Ans

Adithya Nataraj
Jan 8, 2014

Since infinite number of square roots are there, we approach the prob like this : 1. Square on both sides 2 . x + infinitely long term = 25 3 . Since, infinitely long term = 5 4 . x + 5 = 25 5 . x = 20

Ranjit Kumar
Jan 7, 2014

The above problem can be written as sqrt{x+5} = 5; or, x+5 = 25; x= 25-5; or x = 20

Harsh Parasramka
Jan 6, 2014

Squaring both sides, we get x+ \sqrt{x+\sqrt{x+...}} = 25. Using the given equation, x+5=25. Hence, x=20.

Abhishek Sanghai
Jan 5, 2014

at first consider the whole expression to be y=5 next ,square both sides..we get. x+y=25 since y=5 so x=20

Nitin Gupta
Jan 5, 2014

\sqrt x+5=5

squaring both side

x+5=25

x=20

Anil Chik
Jan 5, 2014

given root of(x+<x+.....)=5 implies root of(x+5)=5 implies x+5=25 then we get x=20

Satyajit Maity
Jan 5, 2014

square the both side of the given equation and then we get that x+5 = 5^2 =25 implies that x=20

Luke Limbo
Jan 2, 2014

Based on my analysis, on every radicand must be equal to 25 to become 5, if you subtract 5 from 25 it become 20, so that your x is 20.

Soubhik Paul
Jan 2, 2014

squaring both sides: x+5=25; x=20

Jai Sreenivasan
Jan 1, 2014

let the left side of the equation be =a squaring on both sides we get x+a=25 as a=5 x=20

Adriel Matias
Dec 29, 2013

√(x+√(x+√(x+⋯)) ) = 5

[√x+√(x+√(x+⋯)) ]^2= [5]^2

Let 5 be them:

x+(5)=25 x=25-5 x=20

Ajay Rana
Dec 29, 2013

x + 5 = 25, x = 20.

squaring both sides we get: x +y=25 (y=l.h.s.=5 which is the initial eqn.) x+5=25 x=20

Hopefully this does not get lost among the comments. Evaluate sqrt{x+sqrt{x+sqrt{x+...= 1 what is x? Just don't tell me you get x=0. If so I would sure like an explanation.

Peter Michael - 7 years, 5 months ago
Md Saiful Islam
Dec 29, 2013

Here, x + x + . . . . . + . . . . . \sqrt{x+\sqrt{x+.....}+.....} =5

Hence We have,

x + 5 \sqrt{x+5} =5

So, x+5=25

Or, x=20

The limit of x + x + x + . . . \sqrt{x+\sqrt{x+\sqrt{x+...}}} is 5

Hence, we have

x + 5 = 5 \sqrt{x+5}=5 .

Solving for x gives x = 20 \boxed{x = 20}

Could you please elaborate on how you solved the Limit?

Aaditya Geed - 7 years, 5 months ago
Azizul Islam
Dec 28, 2013

25-5=20

M Ali
Dec 28, 2013

sqrt(x + sqrt(...))=5 x+sqrt(...)=25 5=sqrt(...) x+sqrt(...)-sqrt(...)=25-5=20

Lakshay Nagpal
Dec 28, 2013

it is infinitely long , so if we take out one x , it would not affect the whole series as the series is infinitely long. so the remaining series becomes exactly the same series which is equal to 5.

so therefore

5= Vx+5 squaring and solving , we get x=20

( x + ( x + ( x + . . . ) ) ) = 5 ( x + 5 ) = 5 x + 5 = 25 x = 20 \sqrt(x+\sqrt(x+\sqrt(x+...)))=5\Leftrightarrow\sqrt(x+5)=5\Leftrightarrow x+5=25\Leftrightarrow x=20

James Jusuf
Dec 27, 2013

Notice that x + 5 = 5 \sqrt{x+5}=5 . Squaring both sides, we get x + 5 = 25 x+5=25 so x = 20 x=\fbox{20} .

Jatin Valecha
Dec 27, 2013

tke infinite =5 .. what we left is x+5=5^2 ; 20

Anthony Flores
Dec 27, 2013

5=sqrt(x+5) => 25=x+5=> 20=x.

Omar Paladines
Dec 27, 2013

We define the expression with roots to be S -> S=5. By squaring both sides we get x+S=25, and thus x=25-5=20

Yash Jakhotiya
Dec 27, 2013

square both sides... Now the eqn becomes x+5=25 ( putting given eqn again) So, x=20

Frederick Corpuz
Dec 27, 2013

square both sides 25=x+5

Let x + x + x + \sqrt{x+\sqrt{x+\sqrt{x+\cdots}}} denote the infinite nested radicals.

We have x + x + x + = 5 x + x + x + x + = 5 2 \sqrt{x+\sqrt{x+\sqrt{x+\cdots}}} = 5 \Rightarrow x + \sqrt{x+\sqrt{x+\sqrt{x+\cdots}}} = 5^2 . But because x + x + x + = 5 \sqrt{x+\sqrt{x+\sqrt{x+\cdots}}} = 5 , we can substitue it back and find that x + 5 = 25 x = 20. x + 5 = 25 \Rightarrow \boxed{x = 20.}

Yup you got the trick

Nit Jon - 7 years, 5 months ago

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I just Wikipedia'd "nested radicals".

Guilherme Dela Corte - 7 years, 5 months ago

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