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Question -

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.



Answer -

Let the three numbersbe a/r, a, ar

So, according to thequestion

a/r + a + ar = 39/10 …equation (1)

a/r × a × ar = 1 …equation (2)

From equation (2) weget,

a3 = 1

a = 1

From equation (1) weget,

(a + ar + ar2)/r= 39/10

10a + 10ar + 10ar2 =39r … equation (3)

Substituting a = 1 in3 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 equationwill be,

1/(2/5), 1, 1×2/5 or1/(5/2), 1, 1×5/2

5/2, 1, 2/5 or 2/5, 1,5/2

 The threenumbers are 2/5, 1, 5/2

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