SOLUTION: How do you find the sum of the finite arithmetic series of 9, 16, 23, 30, ..., 100? Thank you

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Question 178522: How do you find the sum of the finite arithmetic series of 9, 16, 23, 30, ..., 100?
Thank you
Answer by Mathtut(3670) About Me  (Show Source):
You can put this solution on YOUR website!
I think I finally figured this one out.
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9 is added to each progressive term and 7's increased by the summation of the number of terms in the sequence The 7's part of this sequence is just a play on Gauss's summation formula.
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so
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9(n+1)+7n(n+1)/2. n is the number of terms . You can figure the number of terms by dividing whatever the number of the last term is and subtracting 9 ,then dividing by 7....such as 100 is the last term so subtract 9 and divide by 7...answer is 13......so n would equal 13
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9(13+1)+7(13)(14)/2=126+637=highlight%28763%29
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you can rewrite the formula to
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(n+1)(9+(7n/2))=%28n%2B1%29%2818%2B7n%29%2F2