How many terms of the AP: 9,17,25,.... must be taken to give a sum of

1.	How many terms of the AP: 9,17,25,.... must be taken to give a sum of 636
Answer» First term of A.P. = 9 , common difference = 8if sum of n terms 636, Sn = (n/2) [ 2×9 + (n-1)8 ] = 636Sn = n [ 10+ 8n] = 12728n2 +10n -1272 = 04n2 +5n -636 = 0By factorising LHS,(4n+53)(n-12) = 0hence n = 12 12 12 12 terms should be taken 12

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