SOLUTION: Find the sum of the first 12 terms of the arithmetic series. -5 + 0 + 5 +10 + ...

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Question 142143: Find the sum of the first 12 terms of the arithmetic series.
-5 + 0 + 5 +10 + ...
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=12 <--- # of terms
a=-5 <--starting term
r=5 <--- common difference between terms

s=12%2A%28-5%29%2B%2812%2812-1%295%29%2F2
=-60%2B330=270

We can check by iterating every term:
-5+0+5+10+15+20+25+30+35+40+45+50=270.