What is the sum of the first 26 terms of the arithmetic series?

The first term is 7, and the common difference is 4. We know this because 11-7=15-11=4. So the nth term is going to be 7+4(n-1). The 26th term will be 7+4(26-1)=7+100=107. The sum of the series is going to be (107+7)*13=1482.

A(1) =7
d=11-7= 4
a(26)= a(1)+d(n-1)= 7+4(26-1)=7+100=107

S(n)=  (n/2)*(a(1) +a(n))
S(26)=(26/2)*(7+107)=13*114=1482
S(26)=1482
1482

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