SOLUTION: Can you please show me how to work this out? I would really be thankful!! What is the sum of the arithmetic sequence 3, 9, 15..., if there are 26 terms? 1,452 1,728

Question 881938: Can you please show me how to work this out? I would really be thankful!!
What is the sum of the arithmetic sequence 3, 9, 15..., if there are 26 terms?
1,452
1,728
2,028
2,268

Found 2 solutions by ewatrrr, jim_thompson5910:
Answer by ewatrrr(24785) (Show Source): You can put this solution on YOUR website!
{3, 9, 15, ...}
Arithmetic Sequence: d = 6
Note typo here
a[26] = 3 + 25d = 3 +

S[26] = 26(3 + 153)/2 = 2,208
Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!
The common difference is 9 - 3 = 6. So d = 6

First term: a1 = 3

Sn = n*(a1 + an)/2

Sn = n*(a1 + a1 + (n-1)*d)/2

Sn = n*(2*a1 + (n-1)*d)/2

S26 = 26*(2*a1 + (26-1)*d)/2 ... plug in n = 26

S26 = 26*(2*3 + (26-1)*d)/2 ... plug in a1 = 3

S26 = 26*(2*3 + (26-1)*6)/2 ... plug in d = 6

S26 = 2,028

The sum of the first 26 terms is 2,028
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