SOLUTION: Find the sum of the first nine terms of the arithmetic series. 2 - 5 - 12 - 19 - ...

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Question 142142: Find the sum of the first nine terms of the arithmetic series.
2 - 5 - 12 - 19 - ...
Answer by nabla(475) About Me  (Show Source):
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The sum of the first n terms of an arithmetic progression is na%2B%28n%28n-1%29r%29%2F2.
n=9 <--- # of terms
a=2 <--starting term
r=-7 <--- common difference between terms

s=9%2A%282%29%2B9%289-1%29%28-7%29%2F2
=18-252=-234

We can check by iterating every term:
2-5-12-19-26-33-40-47-54=-234