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The sum of first three terms of a G.P. is 13/12, and their product is – 1. Find the G.P. |
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Answer» Let the three numbers be a/r, a, ar So, according to the question a/r + a + ar = 13/12 … equation (1) a/r × a × ar = -1 … equation (2) a3 = -1 a = -1 From equation (1) we get, (a + ar + ar2)/r = 13/12 12a + 12ar + 12ar2 = 13r … equation (3) Substituting a = – 1 in equation (3) we get 12( – 1) + 12( – 1)r + 12( – 1)r2 = 13r 12r2 + 25r + 12 = 0 12r2 + 16r + 9r + 12 = 0… equation (4) 4r (3r + 4) + 3(3r + 4) = 0 r = -3/4 or r = -4/3 Now the equation will be -1/(-3/4), -1, -1×-3/4 or -1/(-4/3), -1, -1×-4/3 4/3, -1, 3/4 or 3/4, -1, 4/3 ∴ The three numbers are 4/3, -1, 3/4 or 3/4, -1, 4/3 |
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