What is the formula for these sequence? ______________1 _____________2  3 ___________4    5    6 _____________... 46  47  48  49  50  51  52  53  54  55

3 Answers

oldnick's profile pic

oldnick | (Level 1) Valedictorian

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You can apply Gauss system, this is a triangle  , let's write  firsts rows:


                        2   3

                     4   5    6

                7     8    9   10

             11  12   13  14  15


Now we concern on the last number of any row giving them a "name" as the first one.

Note than  every last number of the n-th row  is the sum of the number row with previous ones :

I.e:   3, last number of second row , is sum of 1, (first row) and 2(second row ).So the last number on third row, 6, si sum of 1,2,3, the numbers of the first three rows and so on,

Designed with  r, the number of row, you can get the last number of ,with the  sum of r numbers row.

Gauss found,when was still a young one, the formula to find sum of r integers in natural sequences.

`S_r= (r^2+r)/2`

Since numbers of every rows running on integer sequence too, they start form the next of previous sequence number sum, to the sum of current row number.

I.e  the r row end with  `(r^2+r)/2`

Row r+1, start with the next ntegers in natural sequence, that is:

`(r^2+r)/2+1`  to the number stands for sum of r rows number:

`((r+1)^2+(r+1))/2` `=((r+1)(r+2))/2` 

So the sqeuence you are searching for, is composed by row  actually contents all integers in the interval :

`r- row:` `[((r+1)^2-(r-1))/2; ((r+1)(r+2))/2]`


beniglesia's profile pic

beniglesia | (Level 1) eNoter

Posted on

What is the formula for these sequence?


                       2  3

                   4    5    6


46  47  48  49  50  51  52  53  54  55

pramodpandey's profile pic

pramodpandey | College Teacher | (Level 3) Valedictorian

Posted on

A sequesnce is defined as

`{S_n}`  ,

where`S_n`  is the  `n^(th)` term of the sequence and have  n terms.

First term of the `S_n`  is

`a_n =a_(n-1)+(n-1)`

where `a_(n-1) ` is first term of `S_(n-1)` , and  `n^(th)`  terms of `S_n`  is


. Also the first term of `S_1`   is `a_1=1`