2016th edition

We have a peculiar series in which a square is written first and then the number of consecutive numbers that follow the square (including the square) is equal to the square divided by its square root.

If the series starts from 1 as { \{ 1, \, 4, 5, \, 9, 10, 11, \, 16, 17, 18, 19, ... } \} , then what is the 2016th number in the series?


The answer is 4031.

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1 solution

Ashish Menon
Jun 6, 2016

Lets divide the sequence into segments as follows:-
1 4 , 5 9 , 10 , 11 16 , 17 , 18 , 19 \color{#3D99F6}{1} \ | \ \color{#3D99F6}{4},5 \ | \ \color{#3D99F6}{9},10,11 \ | \ \color{#3D99F6}{16},17,18,19 \cdots .

So, 1 1 ,is the first term in the sequence.
4 4 is the third term.
9 9 is the sixth term.
16 16 is the tenth term.

So, n 2 n^2 is the ( ( i = 1 n i ) ( n 1 ) ) th {\left(\left(\displaystyle \sum_{i = 1}^n i\right) - (n - 1)\right)}^{\text{th}} term of sequence.

So, let x 2 x^2 be the first square in the 2016 th {2016}^{\text{th}} segment.
Then i = 1 x i = 2016 x ( x + 1 ) 2 = 2016 x 2 + x 4032 = 0 x 2 + 64 x 63 x 4032 = 0 ( x + 64 ) ( x 63 ) = 0 x = 64 ; x = 63 \displaystyle \sum_{i = 1}^x i = 2016\\ \dfrac{x(x + 1)}{2} = 2016\\ x^2 + x - 4032 = 0\\ x^2 + 64x - 63x - 4032= 0\\ (x + 64)(x - 63) = 0\\ x = -64 ; x = 63 .

But, we neglect x = 64 x = -64 , because i = 1 64 i \displaystyle \sum_{i = 1}^{-64} i does not exist.
So, x = 63 x = 63 .
So, the perfect square of the 2016 th {2016}^{\text{th}} segment is 63 2 = 3969 {63}^2 = 3969 .
So, the 2016 th {2016}^{\text{th}} term of the sequence is 3979 + 62 = 4031 3979 + 62 = \color{#3D99F6}{\boxed{4031}}

Very nice :). Hats off to you and your solutions. :)

Abhay Tiwari - 5 years ago

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Jeez! Thanks!

Ashish Menon - 5 years ago

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