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Sum of first 55 terms in an A.P. is 3300, find its 28th term. |
Answer» For an A.P., let a be the first term and d be the common difference. S55 =3300 …[Given] Since, Sn = n/2 [2a + (n – 1)d] ∴ S55 = 55/2 [2a + (55 – 1)d] ∴ 3300 = 55/2 (2a + 54d) ∴ 3300 = 55/2 x 2(a + 27d) ∴ 3300 = 55(a + 27d) ∴ a + 27d = 3300/5 ∴ a + 27d = 60 ....(i) Now, tn = a + (n - 1)d ∴ t28 = a + ( 28 - 1)d = a + 27d ∴ t28 = 60 ... [Frtom (i)] ∴ 28th terms of the A.P is 60. |
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