Question

How do I find the equation of a geometric sequence?

Answer 1

The general formula for an nth term of a geometric sequence is $a{{r}^{n-1}}$. Here, a is the first term of the geometric series, and r is the common ratio of the series. We can find the common ratio by taking the ratio of a term with its previous term. By substituting the values for a, r, and n we can find the desired term that we want. We will take an example to make things more understandable.

Complete step by step solution:
We are given the infinite geometric series $25,75,225,675,...$. Here, the first term is 25, so $a=25$. To find the common ratio, we need to take a ratio of a term with its previous term. Hence, we get the ratio as
$r=\dfrac{75}{25}$,
Here, numerator and denominator have 25 as their highest common factor, cancelling out the common factors, we get $r=3$.
Now, we have the first term and the common ratio. Substituting their values in the formula for the nth term of an geometric series. We get
$a{{r}^{n-1}}$
$25\times {{3}^{n-1}}$
By substituting the different values of n, we can get the terms of the geometric progression

Note: For a general geometric series the formula for the sum of n terms is, $\dfrac{a\left( 1-{{r}^{n}} \right)}{1-r}$ for $\left| r \right|<1$, and $\dfrac{a\left( {{r}^{n}}-1 \right)}{r-1}$ for $\left| r \right|>1$. We can find the sum of infinite series only if the absolute value of the common ratio is less than one, that is $\left| r \right|<1$.
We can derive the formula for infinite geometric series as,
$\displaystyle \lim_{n \to \infty }\dfrac{a\left( 1-{{r}^{n}} \right)}{1-r}$
As $\left| r \right|<1$, we can say that $r^{n} \to 0$ . Using this in the above limit, we get the summation formula as
$\displaystyle \lim_{n \to \infty }\dfrac{a\left( 1-{{r}^{n}} \right)}{1-r}=\dfrac{a\left( 1-0 \right)}{1-r}=\dfrac{a}{1-r}$