SOLUTION: How many terms of the A.P 9,17,25,...Must be taken to give a sum of 636?

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Question 1070092: How many terms of the A.P 9,17,25,...Must be taken to give a sum of 636?
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
d=8
a%5B1%5D=9
.
.
S%5Bn%5D=%28n%2F2%29%282%2Aa%5B1%5D%2B%28n-1%29d%29
Substituting,
%28n%2F2%29%282%2A9%2B%28n-1%298%29=636
n%2818%2B8n-8%29=1272
%288n%2B10%29n=1272
%284n%2B5%29n=636
4n%5E2%2B5n-636=0
%28n-12%29%284n%2B53%29=0
We're only interested in the integer solution.
n-12=0
n=12