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Find the sum of given terms: -81 + 82 + 83 + …. + 89 + 90 |
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Answer» Given series 81 + 82 + 83 + …. + 89 + 90 ∴ Tn = I = a + (n − 1)d ∴ 90 = 81 + (n − 1). 1 (here, a = 81, I = 90 and d = 1) ⇒ n = 9 Now, Sn = \(\frac{n}{2}\)[2a + (n − 1)d] = \(\frac{10}{2}\)[2 × 81 + (10 − 1) × 1] = 5[162 + 9] = 5 × 171 = 855 |
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