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If the ratio of the 11h term of an AP to its 18th term is 2:3, find theratio of the sum of the first five terms to the sum of its first 10 terms |
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Answer» Solution:-(a + 10d)/(a + 17d) = 2/33a + 30d = 2a + 34d3a - 2a = 34 - 30a = 4da₅ = a + 4d= 4d + 4da₅ = 8da₂₁ = a + 20d= 4d + 20da₂₁ = 24dSo, a₅ : a₂₁ = 8 : 24 = 1 : 3The ratio of the sum of the first 5 terms to the sum of the first 21 terms.S₅ = 5/2(8d + 4d)= 60d/2S₅ = 30dS₂₁ = 21/2(8d + 20d)= 588d/2S₂₁ = 294dSo,S₅ : S₂₁ = 30d : 294d= 5 : 49 hit like if you find it useful |
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