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CBSE Class 10 Answered

How many terms of an AP 9,17,25.... must be taken to give a sum of 636?
Asked by sumitkain2016 | 24 Feb, 2019, 08:05: PM
answered-by-expert Expert Answer
First term of A.P. = 9 , common difference = 8
 
if sum of n terms 636,  Sn = (n/2) [ 2×9 + (n-1)8 ] = 636
 
Sn = n [ 10+ 8n] = 1272
 
8n2 +10n -1272 = 0  
 
4n2 +5n -636 = 0
 
By factorising LHS, (4n+53)(n-12) = 0
 
hence n = 12
Answered by Thiyagarajan K | 24 Feb, 2019, 08:25: PM
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