SOLUTION: what is the sum of the series 6+9+12 ... +30

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Question 447497: what is the sum of the series 6+9+12 ... +30
Answer by ewatrrr(24785) About Me  (Show Source):
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Hi

Arithmetic Series d = 3

6+9+12 ... +30

a%5Bn%5D=a%5B1%5D%2B%28n-1%29%2Ad

        6+(n-1)*3 = 30  |finding the correct number of terms

            3n = 27

             n = 9 



S%5Bn%5D=%28n%2F2%29%282%2Aa%5B1%5D%2B+%28n-1%29d%29

S%5B9%5D=%289%2F2%29%282%2A6%2B+8%2A3%29 = (9/2) * 36 = 162