Week 2 – Precalc 11 – Ruben's Blog

This week in math we learned how to find the sum of a geometric sequence. To do this you have to use the formula:

$s=\frac{a(r^n -1)}{r-1}$

so if you were given the geometric sequence 6, 24, 96, 384… and you want to find $s_{15}$ you would do the following

$\frac{24}{6}$ which means r= 4

once you know what r is you would use the formula shown above

$s_{15}=\frac{6(4^{15}-1)}{4-1}$

$s_{15}=\frac{6(1,073,741,824-1)}{4-1}$

$s_{15}=\frac{6(1,073,741,823)}{4-1}$

$s_{15}=\frac{6,442,450,938}{4-1}$

$s_{15}=\frac{6,442,450,938}{3}$

$s_{15}=2,147,483,646$

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