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A geometric sequence goes from one term to the next by always multiplying or dividing by the constant value except 0. The constant number multiplied (or divided) at each stage of a geometric sequence is called the common ratio (r).
A geometric series is the sum of an infinite number of terms of a geometric sequence.
A geometric series is convergers if |r| < 1.
A geometric series is diveres if |r| > 1.
Calculate the common ratio:
[tex]r=\dfrac{18}{27}=\dfrac{18:9}{27:9}=\dfrac{2}{3}\\\\r=\dfrac{12}{18}=\dfrac{12:6}{18:6}=\dfrac{2}{3}\\\\r=\dfrac{8}{12}=\dfrac{8:24}{12:4}=\dfrac{2}{3}[/tex]
[tex]\left|\dfrac{2}{3}\right| < 1[/tex]
Therefore exist the sum.
Formula of a sum of a geometric series:
[tex]S=\dfrac{a_1}{1-r},\qquad|r| < 1[/tex]
Substitute:
[tex]a_1=27,\ r=\dfrac{2}{3}[/tex]
[tex]S=\dfrac{27}{1-\frac{2}{3}}=\dfrac{27}{\frac{1}{3}}=27\cdot\dfrac{3}{1}=81[/tex]
[tex]\huge\boxed{S=81}[/tex]