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The product of three numbers in G.P. is 125 and the sum of their products taken in pairs is 87 1/2. Find them. |
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Answer» Let the three numbers be a/r, a, ar So, according to the question a/r × a × ar = 125 … equation (1) a3 = 125 a = 5 a/r × a + a × ar + ar × a/r = 87 1/2 a/r × a + a × ar + ar × a/r = 195/2 a2/r + a2r + a2 = 195/2 a2 (1/r + r + 1) = 195/2 Substituting a = 5 in above equation we get, 52 [(1 + r2 + r)/r] = 195/2 1 + r2 + r = (195r/2 × 25) 2(1 + r2 + r) = 39r/5 10 + 10r2 + 10r = 39r 10r2 – 29r + 10 = 0 10r2 – 25r – 4r + 10 = 0 5r(2r - 5) – 2(2r -5) = 0 r = 5/2, 2/5 So G.P is 10, 5, 5/2 or 5/2, 5, 10 ∴ The three numbers are 10, 5, 5/2 or 5/2, 5, 10 |
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