Endless Rooting

Algebra Level 1

What would be the answer of: 5 5 5 5 5.............................. = ? \sqrt { 5\sqrt { 5\sqrt { 5\sqrt { 5\sqrt { 5.............................. } } } } } =?


The answer is 5.

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15 solutions

Anik Mandal
Sep 8, 2014

5 5 5........ \sqrt{5 \sqrt{5\sqrt{5........}}}

0r, 5 x = x \sqrt{5x}=x

Or, 5 x = x 2 5x=x^{2}

Or, x = 5 x=5

why cant x be 0 too? it is also a root of the equation..

Abhishek Gorai - 6 years, 9 months ago

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Though x can be 0 from the Quadratic above, from the given value it is evident that x > 0

Suresh Bala - 6 years, 8 months ago

If you take sqrt(5sqrt(5sqrt(5sqrt(5sqrt(5sqrt(5... on a calculator, you'll find the answer approaches 6. Could someone explain?

Julien Bongars - 6 years, 9 months ago

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I'm so handsome

Jade De Guzman - 6 years, 5 months ago

Does it? It approached 5^1 for me...

Anonymous Person - 6 years, 8 months ago

seriously?

Đỉnh Khang Diệp - 5 years, 9 months ago

I agree it was a pretty intimidating problem.......

Ritom Gupta - 6 years, 7 months ago

not a complete solution

Zeeshan Ali - 6 years, 1 month ago

Well done, bro, I mean to Anik.

Gourab Roy - 6 years ago

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Thanks Gourab!

Anik Mandal - 6 years ago

This solution gives me a great notion about infinite series! Thanks!

Koh Hui Soon - 6 years, 5 months ago
John M.
Sep 23, 2014

5 5 5 5... = ? \sqrt{5\sqrt{5\sqrt{5\sqrt{5...}}}}=?

5 5 5 5... = ? 2 \Rightarrow 5\sqrt{5\sqrt{5\sqrt{5...}}}=?^2

5 ? = ? 2 \Rightarrow 5?=?^2

5 = ? \Rightarrow \boxed{5=?}


Or,

5 ? = ? 2 5?=?^2

5 ? ? 2 = 0 \Rightarrow 5?-?^2=0

? ( 5 ? ) = 0 \Rightarrow ?(5-?)=0

? = 0 , 5 \Rightarrow ?=0,5

Since ? 0 ?\neq 0 ,

? = 5 \boxed{?=5} .


(In math, it is generally more proper to solve by factoring rather than canceling like in the first solution. This is because canceling sometimes leads to loss of roots. And even though the root lost in this case was a false one, sometimes it may be a true one.)


GENERAL CASE:

N N N N . . . = N \boxed{\sqrt{N\sqrt{N\sqrt{N\sqrt{N...}}}}=N}

Proof:

N N N N . . . = N \sqrt{N\sqrt{N\sqrt{N\sqrt{N...}}}}=N

N N N N . . . = N 2 \Rightarrow N\sqrt{N\sqrt{N\sqrt{N...}}}=N^2

N N = N 2 \Rightarrow N\cdot N=N^2

N 2 = N 2 \Rightarrow \boxed{N^2=N^2} ,

N \forall N .

Incorrect way to prove something. You assumed that what need to be proved is true.

Like: Proof of -1 = 1: 1 = 1 ( 1 ) 2 = 1 2 1 = 1 -1=1\\~\\(-1)^2=1^2\\~\\ \boxed{1=1}

See how it can go wrong?

Micah Wood - 6 years, 5 months ago

That is not right, because in the final step you get: N × N = N 2 . N \times \sqrt{N} = N^{2}. Then, N = N . \sqrt{N} = N. Finally, N = 0 and N = 1...

Dave Incarnate - 6 years, 6 months ago

makes sense since its an infinite sequence. Of course then it should also be possible to have the answer as 1.

Rubayat Hyder - 6 years, 5 months ago
Finn Hulse
Sep 23, 2014

Let the expression be equal to x x . Then squaring both sides gives x 2 = 5 x x^2=5x . If you don't see why, notice how by squaring x x adds a coefficient of 5 5 to x x . Solving, we have x = 5 x=5 .

because of the expression in spread over infinite so i can assume the value of given expression is equal to =x and squaring on both side i will get the expression x^2=5(and in square root of the expression due to the infinite value it is assume that the value is equal to x) and solve these equation will get answer appropriate.................thats is if you have any quaries you can call me 9616667833..................so thanks friends...............

Avadhendra Yadav - 6 years, 5 months ago

Whatcha doin, boy? Translating my solution to English? Lol

John M. - 6 years, 8 months ago
William Isoroku
Dec 15, 2014

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

Then x 2 = 5 5 5 5.... { x }^{ 2 }=5\sqrt { 5\sqrt { 5\sqrt { 5.... } } }

We already know that 5 5 5.... = x \sqrt { 5\sqrt { 5\sqrt { 5.... } } } =x , so 5 5 5 5.... = 5 x 5\sqrt { 5\sqrt { 5\sqrt { 5.... } } }=5x

It becomes x 2 = 5 x { x }^{ 2 }=5x and x = 0 a n d 5 x=0\quad and\quad 5

So x = 5 \boxed{x=5}

Iain Ross
Dec 15, 2014

If X equal the above equation. Then x^2 will equal 5x. This can be solved through simple algebraic methods to give x=5 or x=0. 0 can be disagreed

Dat solution!

Josh Williams - 6 years, 6 months ago
Ishi Nigam
Dec 18, 2018

Powers in GP, Infinite gp sum =1/1-1/2 =2 Root over 5^2 =5

Zeeshan Ali
Apr 18, 2015

let the given expration be x then we have x=sqrt(5x) or x^2 = 5x ==>x=0,5 hence the expresion is either zero or 5. Upvote iff agree..! :)

Steve Cairns
Dec 30, 2014

Expression = Product over n=1,2,3... of 5^(2^-n) = 5^(Sum over n=1,2,3... of 2^-n) = 5 ^ 1

Souradip Dey
Dec 28, 2014

y=Q Y y=5 Q y(y-5)=0 y=0,5

Assume that 5^1/2(5^1/2 5^1/2 ....= a ------(1) 5(5^1/2 5^1/2 .... = a^2 -------(2) I insert (1) in (2) will be 5a=a^2 So that a^2-5a=0 a(a-5)=0 a=5,0 but a cannot be 0 during question so a must be 5

Mukisa Robbie
Dec 15, 2014

Just made a brilliant thought. if there are five terms and they are all similar to the number of square root symbols, in this case 5, then the answer is 5.

Anna Anant
Dec 15, 2014

y= sqrt(5* sqrt (5* sqrt (5* sqrt ... ... ... =>y^2= 5 y =>y^2-5 y =0 =>y*(y-5)=0 so, y=5

Mohammed Ali
Dec 15, 2014

let root of 5...... be x

as the nested radical continues till infinity, it can be written as=

root of 5 *x = x

5x = x^2

x^2 - 5x =0,,, x(x-5) = 0,

x-5 = 0, x=5

Bibek Kundu
Dec 15, 2014

x=√(5√(5√5) ) ……………∞, x^2=5x x=5

\sqrt { 5\sqrt { 5\sqrt { 5.. } } }

Lets assume: sqrt { 5\sqrt { 5.. } } = x 5x = { x }^{ 2 } { x }^{ 2 }-5x = 0 x(x-5) = 0 x = 0,5

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