Q26 of 34 Page 1

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

9,17,25, .......


Sn = 636


a = 9


d = a2 – a1


= 17 – 9 = 8


Sn = [ 2a + (n – 1)d]


Sn = [ 2a + (n – 1)d]


636 = [ 2 × 9 + (n – 1) 8]


1272 = n [ 18 + 8n – 8]


1272 = n [10 + 8n]


8n2 +10n – 1272 = 0


4n2 + 5n – 636 = 0


n = −


n = −


n =


n = n =


n = 12 n =


n = 12 (since n cannot be negative)


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