Amazing Nested Radical 3

Algebra Level 2

1 + 4 + 4 2 + 4 3 + 4 4 + = ? \sqrt{1+\sqrt{4+\sqrt{4^2+\sqrt{4^3+\sqrt{4^4+\cdots}}}}} = \ ? \


The answer is 2.

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3 solutions

Rishabh Jain
Feb 1, 2016

x + 1 = ( x + 1 ) 2 x+1=\sqrt{(x+1)^2} = x 2 + 2 x + 1 =\sqrt{x^2+\color{#D61F06}{2x+1}} = x 2 + ( 2 x + 1 ) 2 =\sqrt{x^2+\sqrt{\color{#D61F06}{(2x+1)^2}}} = x 2 + 4 x 2 + 4 x + 1 =\sqrt{x^2+\sqrt{4x^2+\color{#20A900}{4x+1}}} = x 2 + 4 x 2 + ( 4 x + 1 ) 2 =\sqrt{x^2+\sqrt{4x^2+\sqrt{\color{#20A900}{(4x+1)^2}}}} = x 2 + 4 x 2 + 16 x 2 + 8 x + 1 =\sqrt{x^2+\sqrt{4x^2+\sqrt{16x^2+\color{#3D99F6}{8x+1}}}} = =\cdots Put x=1 to get the desired result i.e 2 = 1 + 4 + 4 2 + 4 3 + 4 4 + \Large\color{#624F41}{\boxed{\color{#69047E}{2}}}=\sqrt{1+\sqrt{4+\sqrt{4^2+\sqrt{4^3+\sqrt{4^4+\cdots}}}}}

Has anyone found a method for reversing infinitely nested square roots? I've seen many solutions where an expression is shown to be equivalent to the question, but I've never seen anyone show how you could get from a nested radical back to a simple expression.

John Frank - 5 years, 4 months ago

Why x + 1 x+1 ?

Akshat Sharda - 5 years, 4 months ago

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Mathematical intuition... You might need to see his previous problems.... They are improvisations of the same concept (yet they are too good)... :D ...Anyways what idea did you used to solve this problem?

Rishabh Jain - 5 years, 4 months ago

Yeah, I'm also curious as to how he got x + 1 x+1 as the function he should use...

Manuel Kahayon - 5 years, 4 months ago

You may have a look at my solution which is for (x+a) instead of (x+1), thus is a better generalization of the presented nested radical. Thank You

Mrigank Shekhar Pathak - 4 years, 4 months ago

We know ( x + a ) = x 2 + a 2 + 2 a x = a 2 + x ( x + 2 a ) = a 2 + x 4 a 2 + x ( x + 4 a ) = a 2 + x 4 a 2 + x 16 a 2 + x ( x + 8 a ) = a 2 + x 4 a 2 + x 16 a 2 + x 64 a 2 + x . . . Now, setting x=a=1 we get, 2 = 1 + 4 + 16 + 64 + 256 + . . . \text{We know} \\ (x+a) \\ = \sqrt{x^2 + a^2 + 2ax} \\ = \sqrt{a^2 + x(x+2a)} \\ = \sqrt{a^2 + x\sqrt{4a^2 + x(x+4a)}} \\ = \sqrt{a^2 + x\sqrt{4a^2 + x\sqrt{16a^2 + x(x+8a)}}} \\ = \sqrt{a^2 + x\sqrt{4a^2 + x\sqrt{16a^2 + x\sqrt{64a^2 + x\sqrt{...}}}}} \\ \text{Now, setting x=a=1 we get,} \\ 2=\sqrt { 1+\sqrt { 4+\sqrt { 16+\sqrt { 64+\sqrt { 256+\sqrt { ... } } } } } } Try out these crazy nested radicals!

Giorgos K
Feb 1, 2016

How is this? 4x+1=(4x+1)^2

2nd green expression in your solution

Something isn t right here. If I put x=1 in the 2nd red expresion, that doesn t make the the equation of the problem

He did inverse calculations, which cancel out. Rooted and squared it, and the number didn't change.

Iosi Ginerman - 5 years, 4 months ago

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