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ICSE Class 10 Answered

The sum of first 7 terms of an AP is 49 and that of first 17 terms of it is 289 . Find the sum of first n terms .
Asked by debabratap261 | 23 Mar, 2019, 08:49: AM
answered-by-expert Expert Answer
Sum of 7 terms = (7/2) [ 2a + 6 d ] = 49  or        a+3d = 7  ....................(1)
 
Sum of 17 terms = (17/2) [ 2a + 16 d ] = 289 or a+8d = 17 ...................(2)
 
By solving eqn. (1) and (2), we get a = 1, d = 2
 
Hence sum of n terms  (n/2)[2+(n-1)2 ] = n2
Answered by Thiyagarajan K | 23 Mar, 2019, 09:30: AM
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