How many terms of the AP 9, 17, 25, ... must be taken so that their sum is 636?

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Accepted Answer

12. a=9, d=8, S=636. 636 = n/2[18 + 8(n-1)] => 1272 = n(8n + 10) => 8n^2 + 10n - 1272 = 0 => 4n^2 + 5n - 636 = 0. n = -5 + √25 + 101768 = -5 + √102018 = -5 + 101/8 = 96/8 = 12.

How many terms of the AP 9, 17, 25, ... must be taken so that their sum is 636?

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