[tex]\bf \textit{sum of an infinite geometric serie}\\\\
\stackrel{for~~|r|\ \textless \ 1}{S=\sum\limits_{i=0}^{\infty}~a_1r^i\implies \cfrac{a_1}{1-r}}\qquad
\begin{cases}
a_1=\textit{first term's value}\\
r=\textit{common ratio}\\
----------\\
a_1=42\\
r=\frac{3}{4}
\end{cases}
\\\\\\
S=\cfrac{42}{1-\frac{3}{4}}\implies S=\cfrac{42}{\frac{1}{4}}\implies S=164[/tex]
bearing in mind that, the geometric sequence is "convergent" only when |r|<1, or namely "r" is a fraction between 0 and 1.
