SOLUTION: Find the sum of the series described by a1=-18,d=3 and n=20

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Question 481803: Find the sum of the series described by a1=-18,d=3 and n=20
Answer by ewatrrr(24785) About Me  (Show Source):
You can put this solution on YOUR website!
 

Hi,

a1=-18,d=3 and n=20

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

S%5B20%5D+=%2820%2F2%29%282%2A-18+%2B+%2819%293%29=+-90