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The common difference of an A.P is -2. Find its sum, if first term is 100 and last term is -10. |
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Answer» Given, a = 100, d = -2, tn = -10 Using, tn = a + (n - 1)d -10 = 100 + (n - 1)(-2) -10 = 100 - 2n + 2 2n = 100 + 2 + 10 2n = 112 n = 112/2 n = 56 Here 56th term is -10 Number of terms in A.P. are 56 Sn = n/2(a + l) S56 = 56/2 (100 - 10) = 28 x 90 = 2520 |
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