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If the sum of first 7 terms of an AP is 49 and that of first 17 terms is 289, find the sum of its first n terms? (please do reply fast)
Given that,
S 7 = 49
S17 = 289

7 = (a + 3d)
a + 3d = 7     (i)
Similarly,

17 = (a + 8d)
a + 8d = 17     (ii)
Subtracting equation (i) from equation (ii),
5d = 10
d = 2
From equation (i),
a + 3(2) = 7
a + 6 = 7
a = 1

= n 2


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