SOLUTION: What are the steps to solve this question: Find the sum of the first n terms of each arithmetic sequence. -5, -1, 3....; n=17

Question 934335: What are the steps to solve this question:
Find the sum of the first n terms of each arithmetic sequence.
-5, -1, 3....; n=17
Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!
In this case, the first term is a1 = -5 and the common difference is d = 4 (eg: -1 minus -5 = 4)
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Sn = n*(a1 + an)/2

Sn = n*(a1 + a1 + d*(n-1))/2

Sn = n*(2a1 + d*(n-1))/2

Sn = n*(2*(-5) + 4*(n-1))/2

S17 = 17*(2*(-5) + 4*(17-1))/2

S17 = 459
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