if the sum of first 7 terms of an ap is 49 and that of 17 terms is 289,find the sum of first n terms
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given: (2a + 6 d) * 7/2 = 49
=> a + 3d = 7
(2a + 16 d) * 17/2 = 289
=> a + 8 d = 17
so 5 d = 10 => d = 2 => a = 1
series is : 1, 3, 5, 7, 9, ....so sum of n terms : [2 a + (n-1) d] n/2 = = n^2
=> a + 3d = 7
(2a + 16 d) * 17/2 = 289
=> a + 8 d = 17
so 5 d = 10 => d = 2 => a = 1
series is : 1, 3, 5, 7, 9, ....so sum of n terms : [2 a + (n-1) d] n/2 = = n^2
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