Question
In an A.P a9 = 40, then find sum of first 17 terms of
that A.P.Solution
General form of A.P is a, a + d, a + 2d, a + 3d….. Here a9 = 40; so 9th term is a + 8d = 40 then S17 =? ⇒ Sn = (n/2) [2 x a + (n – 1)d] = 0 ⇒ S17 = (17/2) [2a + 16d] ⇒ (17) [a + 8d] ⇒ 17 x 40 = 680
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