Question

1,2,3,4...................tn
tn=?

Answer 1

As we can see the first term is 1, second term is 2 and  so on.

It is an arithmetic progression with a common difference 1 and starting term 1.

a = 1  d = 1

Tn = n    OR,  a + (n-1) d = 1 + (n-1)1 = n

Sum of n terms =  n (n+1) / 2

Answer 2

This is an AP
a = 1 and the common difference, d = 1
nth term:
$t_n = a + (n-1)d$
$t_n = 1 + (n-1)1$
$t_n = 1 + n-1$
$t_n = n$