Question

me ajudem neeesa questão plss

Answer 1

Bom dia

$a_{3} =20\Rightarrow\ a_{3}= a_{1}* q^{2}=20 \\ a_{7}=320\Rightarrow\ a_{7}= a_{1} * q^{6}=320 \\ \\ ( a_{1}* q^{6} ) : ( a_{1}* q^{2} ) =320:20 \\ \\ q^{4} =16\Rightarrow\ q=2 \\ \\ a_{1}* 2^{2}=20\Rightarrow\ a_{1} *4=20\Rightarrow\ a_{1}=5 \\ \\ a_{9}=5* 2^{8} \Rightarrow\ a_{9}=1280$

$S_{n}= \frac{ a_{1}*( q^{n}-1 ) }{q-1} \\ \\ S_{9}= \frac{5*( 2^{9}-1 )}{2-1}= \frac{5*(512-1)}{1} = \frac{5*511}{1} =2555$

Resposta : letra  C    [ 2555 ]