What is the sum of the arithmetic sequence 3, 9, if there are 26 terms?

9 - 3 = 6, 15-9 = 6 the difference is 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

 substitute 26 for n
S26 = 26*(2*a1 + (26-1)*d)/2

substitute 3 for a1

S26 = 26*(2*3 + (26-1)*d)/2

substitute 6 for d

S26 = 26*(2*3 + (26-1)*6)/2  

S26 = 2,028

the correct answer is $7.38

step-by-step explanation: