InterviewSolution
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The ratio of the sum of the first three terms is to that of the first 6 terms of a G.P. is 125 : 152. Find the common ratio. |
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Answer» Given: Sum of G.P of 3 terms is 125 By using the formula, Sum of GP for n terms = a(rn – 1 )/(r – 1) 125 = a (rn – 1)/(r - 1) 125 = a (r3 – 1)/ (r - 1) … equation (1) Now, Sum of G.P of 6 terms is 152 By using the formula, Sum of GP for n terms = a(rn – 1 )/(r – 1) 152 = a (rn – 1)/(r - 1) 152 = a (r6 – 1)/ (r - 1) … equation (2) Let us divide equation (i) by (ii) we get, 125/152 = [a (r3 – 1)/ (r - 1)] / [a (r6 – 1)/ (r - 1)] 125/152 = (r3 – 1)/(r6 – 1) 125/152 = (r3 – 1)/[(r3 – 1) (r3 + 1)] 125/152 = 1/(r3 + 1) 125(r3 + 1) = 152 125r3 + 125 = 152 125r3 = 152 – 125 125r3 = 27 r3 = 27/125 r3 = 33/53 r = 3/5 ∴ The common ratio is 3/5 |
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