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The sum of the first three terms of a G.P. is 39/10, and their product is 1. Find the common ratio and the terms. |
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Answer» Let the three numbers be a/r, a, ar So, according to the question a/r + a + ar = 39/10 … equation (1) a/r × a × ar = 1 … equation (2) a3 = 1 a = 1 From equation (1) we get, (a + ar + ar2)/r = 39/10 10a + 10ar + 10ar2 = 39r … equation (3) Substituting a = 1 in 3 we get 10(1) + 10(1)r + 10(1)r2 = 39r 10r2 – 29r + 10 = 0 10r2 – 25r – 4r + 10 = 0… equation (4) 5r(2r – 5) – 2(2r – 5) = 0 r = 2/5 or 5/2 so now the equation will be, 1/(2/5), 1, 1 × 2/5 or 1/(5/2), 1, 1 × 5/2 5/2, 1, 2/5 or 2/5, 1, 5/2 ∴ The three numbers are 2/5, 1, 5/2 |
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