Ramanujam's Problem

Algebra Level 3

x = 1 + 2 1 + 3 1 + 4 1 + \large x = \sqrt{1+ 2 \sqrt{1+3 \sqrt{1+4 \sqrt{1+ \cdots}}}}

Solve for x x .


The answer is 3.

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2 solutions

Chew-Seong Cheong
Apr 11, 2017

Using Ramanujan's nested radical formula (eqn. 26) below:

x + n + a = a x + ( n + a ) 2 + x a ( x + n ) + ( n + a ) 2 + ( x + n ) a ( x + 2 n ) + ( n + a ) 2 + ( x + 2 n ) x+n+a = \sqrt{ax+(n+a)^2+x\sqrt{a(x+n)+(n+a)^2+(x+n)\sqrt{a(x+2n)+(n+a)^2+(x+2n)\sqrt \cdots}}}

Putting x = 2 x=2 , n = 1 n=1 and a = 0 a=0 , we have:

2 + 1 + 0 = 0 + 1 2 + 2 0 + 1 2 + ( 2 + 1 ) 0 + 1 2 + ( 2 + 2 ) 3 = 1 + 2 1 + 3 1 + 4 \begin{aligned} 2+1+0 & = \sqrt{0+1^2+2\sqrt{0+1^2+(2+1)\sqrt{0+1^2+(2+2)\sqrt \cdots}}} \\ \implies \boxed{3} & = \sqrt{1+2\sqrt{1+3\sqrt{1+4\sqrt \cdots}}} \end{aligned}

That's good :D

Vaibhav Kandwal - 4 years, 2 months ago

Simple enough

Biswajit Barik - 4 years, 2 months ago
Vaibhav Kandwal
Apr 11, 2017

This one is rather a controversial solution:

Hope anyone can provide another solution.

Here, I'm simply expanding the numbers in a particular order. 3 = 9 = 1 + 8 = 1 + ( 2 × 4 ) = 1 + 2 16 3= \sqrt{9} = \sqrt{1+8} = \sqrt{1+(2 \times 4)} = \sqrt{1+2 \sqrt{16}} Now, 16 can be written as 15+1

3 = 1 + 2 1 + 15 = 1 + 2 1 + ( 3 × 5 ) = 1 + 2 1 + 3 25 \Rightarrow 3 = \sqrt{1+2 \sqrt{1+15}} = \sqrt{1+2 \sqrt{1+(3 \times 5)}} = \sqrt{1+2 \sqrt{1+3 \sqrt{25}}}

Again, we can write 25 as 1+24 and 24 can be split as 4 × 6 4 \times 6 and then again 6 can be written as 36 \sqrt{36} and so on!

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