Nested sqrt

Calculus Level 5

Evaluate 2013 + 276 2027 + 278 2041 + 280 2055 + \sqrt{2013+276\sqrt{2027+278\sqrt{2041+280\sqrt{2055+\ldots}}}}


The answer is 285.

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1 solution

Tapas Mazumdar
May 23, 2018

We have

2013 + 276 2027 + 278 2041 + 280 2055 + = ( 7 ) ( 276 ) + ( 9 ) 2 + 276 ( 7 ) ( 276 + 2 ) + ( 9 ) 2 + ( 276 + 2 ) ( 7 ) ( 276 + 2 × 2 ) + ( 9 ) 2 + ( 276 + 2 × 2 ) \sqrt{2013+276\sqrt{2027+278\sqrt{2041+280\sqrt{2055+\ldots}}}} \\ = \sqrt{(7)(276)+ (9)^2 +276\sqrt{(7)(276+2) + (9)^2 + (276+2)\sqrt{(7)(276+2\times 2) + (9)^2 +(276+2\times 2)\sqrt{\ldots}}}}

which can be compared to Ramanujan's infinite nested radical function

F ( n , a , x ) = a x + ( a + n ) 2 + x a ( x + n ) + ( a + n ) 2 + ( x + n ) a ( x + 2 n ) + ( a + n ) 2 + ( x + 2 n ) F(n,a,x) = \sqrt{ax + (a+n)^2 + x \sqrt{ a(x+n) + (a+n)^2 + (x+n) \sqrt{ a(x+2n) + (a+n)^2 + (x+2n) \sqrt{\ldots}}}}

to get a = 7 , n = 2 , x = 276 a=7, n=2, x=276 . The answer to the infinite nested radical function is F ( n , a , x ) = n + a + x F(n,a,x) = n+a+x giving us the answer as 2 + 7 + 276 = 285 2+7+276 = \boxed{285} .

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