The sum of first three terms of a G.P. is 39/10 and their product is 1. Find the common ratio and the terms.
The sum of first three terms of a G.P. is 39/10 and their product is 1. Find the common ratio and the terms.

Let

    \[a/r,\text{ }a,\text{ }ar\]

be the initial three terms of the

    \[G.P.\]

    \[\begin{array}{*{35}{l}} a/r\text{ }+\text{ }a\text{ }+\text{ }ar\text{ }=\text{ }39/10\text{ }\ldots \ldots \text{ }\left( 1 \right)  \\ \left( a/r \right)\text{ }\left( a \right)\text{ }\left( ar \right)\text{ }=\text{ }1\text{ }\ldots \ldots ..\text{ }\left( 2 \right)  \\ \end{array}\]

From

    \[\left( 2 \right),\]

we have

    \[{{a}^{3}}~=\text{ }1\]

Subsequently,

    \[a\text{ }=\text{ }1\]

[Considering genuine roots only]

Subbing the worth of

    \[a\]

in

    \[\left( 1 \right),\]

we get

    \[\begin{array}{*{35}{l}} 1/r\text{ }+\text{ }1\text{ }+\text{ }r\text{ }=\text{ }39/10  \\ (1\text{ }+\text{ }r\text{ }+\text{ }{{r}^{2}})/r\text{ }=\text{ }39/10  \\ 10\text{ }+\text{ }10r\text{ }+\text{ }10{{r}^{2}}~=\text{ }39r  \\ 10{{r}^{2}}~\text{ }29r\text{ }+\text{ }10\text{ }=\text{ }0  \\ 10{{r}^{2}}~\text{ }25r\text{ }\text{ }4r\text{ }+\text{ }10\text{ }=\text{ }0  \\ 5r\left( 2r\text{ }\text{ }5 \right)\text{ }\text{ }2\left( 2r\text{ }\text{ }5 \right)\text{ }=\text{ }0  \\ \left( 5r\text{ }\text{ }2 \right)\text{ }\left( 2r\text{ }\text{ }5 \right)\text{ }=\text{ }0  \\ \end{array}\]

In this manner,

    \[r\text{ }=\text{ }2/5\text{ }or\text{ }5/2\]

In this manner, the three terms of the G.P. are

    \[5/2,\text{ }1\text{ }and\text{ }2/5.\]