What is the sum of $5, 10, 20, 40,..., a_12$ ?

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Answer 1 · 2016-01-10T00:27:09

Use the geometric sum formula ...

$S_n=(a_1-a_(n+1))/(1-r)$

First, find $r$ and $a_13$ ...

$r=10/5=2$

$a_13=5(2)^((13-1))=20480$

$S_12=(a_1-a_(13))/(1-r)=(5-20480)/(1-2)=20475$

hope that helped!

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