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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 тАжequation (1)

a/r ├Ч a ├Ч ar = 1 тАжequation (2)

From equation (2) weget,

a3┬а= 1

a = 1

From equation (1) weget,

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

10a + 10ar + 10ar2┬а=39r тАж equation (3)

Substituting a = 1 in3 we get

10(1) + 10(1)r +10(1)r2┬а= 39r

10r2┬атАУ29r + 10 = 0

10r2┬атАУ25r тАУ 4r + 10 = 0тАж equation (4)

5r(2r тАУ 5) тАУ 2(2r тАУ 5)= 0

r = 2/5 or 5/2

so now the equationwill be,

1/(2/5), 1, 1├Ч2/5 or1/(5/2), 1, 1├Ч5/2

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

тИ┤┬аThe threenumbers are 2/5, 1, 5/2

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