SOLUTION: 1,-2,-5,-8,....,-236 find the sum of ap

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Question 939407: 1,-2,-5,-8,....,-236 find the sum of ap
Answer by ewatrrr(24785) About Me  (Show Source):
You can put this solution on YOUR website!
1,-2,-5,-8,....,-236
d = -3
an = 1 + (n-1)(-3) = -236, -3n = -234, n = 78
....
S%5Bn%5D=%28n%28a%5B1%5D%2Ba%5Bn%5D%29%29%2F2
S%5B78%5D=%2878%281+%2B+%28-236%29%29%29%2F2
S%5B78%5D=39%28-235%29