SOLUTION: Find the sum of the first 12 terms of the arithmetic sequence given by: an (the n after the a is small at sets to the lower right side of the a) =5n. a. 391 b.780 c.390 d.360

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Question 880626: Find the sum of the first 12 terms of the arithmetic sequence given by: an (the n after the a is small at sets to the lower right side of the a) =5n.
a. 391
b.780
c.390
d.360
Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
S = ½(2A1 + (n-1)d)n
where A1=5
d=5
n=12
S = ½(2*5 + (11)5)12
S=6*(10+55)
S=6*65=390