IIT JEE , AIEEE Forum - Mathematics , Algebra

umang

The series of natural numbers divided into groups as follows -

1

2 3

4 5 6

7 8 9 10 and so on

Then what is the sum of numbers in the nth group is ????

unavailable

hi umang

looking the first element of each line

i.e 1,2,4,7

s=1+2+4+7................................tn

s= 1+2+4+7.............................t_n-1..+t _n

0=1+1+2+3+4.............................tn-1-tn

tn=1+((n-1)n)/2

=(n²-- n + 2)/2

same way the last term of the each line

s=1+3+6+......................................tn

s= 1+3+6+.............................t_n-1..+t _n

0=1+2+3+4+...............................tn-1-tn

tn=((n+1)n)/2

nth term =(n²+ n )/2

sum of nth line =(n/2)( t1 + tn )

= (n³+ n)/2

umang

Hey unavailable !

yes , you are perfectly right . I had used the same approach , but was not getting the correct answer .

By the way , whats ur name ???????

umang

Hey !

Pls try the other questions in sequence and series I hav posted !!!!!!

unavailable

Shashank

umang

Hi shashank !

so , u r new to goiit ?????

In which class do u study ????

what r u preparing for ???????

I am in class XI preparing for IITJEE 2008 .

unavailable

same here