Hi, I'm pretty sure by S12 you mean the sum of all the 12 terms in the series 2 + 5 + 8 + .....
Then, first you find the pattern:
5 -2 = 3
8 -5 = 3
This is an arithmetic series with a common difference of 3. Next, we find the last term, or the 12th term.
A12 = A1 + (n - 1)d
A12 = 2 + (12 - 1)3
A12 = 35
Since, we know the first and last terms of the series, we could find the sum by this formula:
S = (n/2)*(A1 + A12)
S = (12/2) * (2 + 35)
S = 222
The answer is 222.
