Floor function 2

Let a n = n 2 104 , a_{n} = \left\lfloor \frac{n^{2}}{104} \right\rfloor, where x \left\lfloor x \right\rfloor gives the largest integer less than or equal to x . x. Then how many distinct numbers are there in the sequence a 1 , a 2 , a 3 , a 103 ? a_{1} , a_{2}, a_{3}, \cdots a_{103}?


The answer is 78.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Joshua Chin
Feb 29, 2016

Now consider putting two consecutive positive integers as a difference of two squares so that the difference is 104, if not around 104. This is also represented as ( n + 1 ) 2 n 2 = 104 { (n+1) }^{ 2 }-{ n }^{ 2 }=104 .

and we get 2 n + 1 = 104 n = 51.5 2n+1=104\\ n=51.5

But n n is not an integer, so let n = 52 n=52 .

Now subbing in n = 52 n=52 gives us ( n + 1 ) 2 n 2 = 105 > 104 { (n+1) }^{ 2 }-{ n }^{ 2 }=105>104

And 105 104 > 1 \frac{105}{104}>1 and this means that the floor function of any term from n = 52 n=52 will have a difference of at least one and thus the terms after n = 52 n=52 are distinct.

And thus, every term before n = 52 n=52 may or may not be distinct. The common difference of each term, 2 n + 1 < 104 2n+1 <104 . But 5 2 2 104 = 26 \frac{52^2}{104}=26 . So this means there are 26 0 + 1 = 27 26-0+1=27 distinct terms in the range 1 n 52 1\le n\le 52 .

Now for the range 53 n 103 53\le n\le 103 , since the common difference of 2 n + 1 > 104 2n+1>104 , each of the term in the range 53 n 103 53\le n\le 103 is distinct, so there are 103 53 + 1 = 51 103-53+1=51 distinct terms in the range 53 n 103 53\le n\le 103 .

Thus, there are 27 + 51 = 78 27+51=78 distinct terms in all.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...