Question

Find the geometric sum: 40 + 20 + 10 + … + 0.078125

Answer 1

$S=40+20+10+\cdots+0.078125$
$S=40\left(1+\dfrac12+\dfrac14+\cdots+\dfrac1{512}\right)$
$S=40\left(1+2^{-1}+2^{-2}+\cdots+2^{-9}\right)$

$2^{-1}S=40\left(2^{-1}+2^{-2}+2^{-3}+\cdots+2^{-10}\right)$

$\implies S-2^{-1}S=40\left(1-2^{-10}\right)$
$\implies\dfrac12S=40\left(1-\dfrac1{1024}\right)$
$\implies S=80\left(1-\dfrac1{1024}\right)=\dfrac{5115}{64}=79.921875$