Question

The sum of the first two terms of an infinite G.P is $5$ and each term is three times the sum of succeeding terms. Find the series.

Answer 1

Hint: From the given conditions in the problem first we will try to find the first term and common ratio of the series using nth term formula which is given by ${T_n} = a{r^{n - 1}}$
Where ${T_n}$ is the nth term, a is the first term and r is the common ratio.

Given that:
The sum of first two term is $5$
$\Rightarrow {T_1} + {T_2} = 5$
$\Rightarrow a + ar = 5$ ………………………….. (1)
And each term is three times the succeeding term
${T_n} = 3({T_{n + 1}} + {T_{n + 2}} + ........\infty )$
Substituting the value of ${T_n}$ in term of a and r
$\Rightarrow a{r^{n - 1}} = 3(a{r^n} + a{r^{n + 1}} + .....\infty )$
Simplifying the above equation we will get
$\Rightarrow 1 = 3r(\dfrac{1}{{1 - r}}) \\\ \Rightarrow 1 - r = 3r \\\ \Rightarrow 4r = 1 \\\ \Rightarrow r = \dfrac{1}{4} \\\$
Substitute the value of r in equation 1, we get
$\Rightarrow a + a(\dfrac{1}{4}) = 5 \\\ \Rightarrow \dfrac{5}{4}a = 5 \\\ \Rightarrow a = 4 \\\$
First term of the G.P. $= a = 4$
Second term of the G.P. $= ar = 4 \times \dfrac{1}{4} = 1$
Third term of the G.P. $= a{r^2} = 4 \times {\left( {\dfrac{1}{4}} \right)^2} = \dfrac{1}{4}$
And so on the G.P continues…..

Hence, the infinite G.P series is
$4,1,\dfrac{1}{4},\dfrac{1}{{16}},...........\infty$

Note: A G.P is a sequence such that any element after the first is obtained by multiplying the preceding element by a constant called common ratio. For solving this type of problem remember the formula of the nth term of a geometric progression series and read the conditions of the question carefully and then start solving for the unknown values whether common ratio or first term.