SOLUTION: Find n if Sn = 63, a1 = 21, and d = -4

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Question 976760: Find n if Sn = 63, a1 = 21, and d = -4
Answer by FrankM(1040) About Me  (Show Source):
You can put this solution on YOUR website!
S%5Bn%5D=%28n%2F2%29%282a%5B1%5D%2B%28n-1%29d%29
let's enter what we were given -
63=%28n%2F2%29%2842-4%28n-1%29%29

63=%28n%2F2%29%2846-4n%29%29
126=%2846n-4n%5E2%29%29
4n%5E2-46n%2B126=0
2n%5E2-23n%2B63=0
I'd use the quadratic equation here, note, the solutions result in 4.5 and 7, but the answer must be an integer.
n=7