Determine an arithmetic sequence knowing the sum Sn if its n first terms is equal to 6n^2 + 7n. For any non zero n
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Start from here: $\displaystyle \frac{n}{2}(2a+(n-1)d)=6n^2+7n$
When n=1 we get the first term. That is 13. When n=2 we get sum of 1st 2 terms = 38 Hence 2nd term= 25 So common difference = 12
Originally Posted by biffboy When n=1 we get the first term. That is 13. When n=2 we get sum of 1st 2 terms = 38 Hence 2nd term= 25 So common difference = 12 That's better than what I said.
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