Question 53424
The difference between 2 terms is 2. 

The nth term is, according to the formula:
{{{a[n] = a[1] + d(n-1)}}}. 

{{{a[100] = 1 + 2(101 - 1) = 201}}}

The sum of the first n terms is, according to the formula:

{{{S[n] = (n/2)(a[1] + a[n])}}}
{{{S[n] = (n/2)(a[1] + a[1] + d(n-1))}}}

{{{S[20] = 10(1 + 1 + 2*19)}}}
{{{S[20] = 400}}}

{{{S[30] = 15(1 + 1 + 2*29)}}}
{{{S[30] = 15*60}}}
{{{S[30] = 900}}}

If you calculate {{{S[n]}}} for n from 1 to 10, you'll see that:
{{{S[n] = n^2}}}