When we add odd numbers:
1 = 1
1 + 3 = 4
1 + 3 + 5 = 9
...
Notice that the sum of n terms is n²
* This is because an odd number is expressed as 2n-1
So we would have 2(1+2+3+...+n) - n = 2[n(n+1)/2] - n = n(n+1) - n = n(n+1-1) = n²
So:
Sum of n terms = 961
Sum of n terms = 31² = n²
n = 31
There are 31 terms in the arithmetic sequence
