Question

Геометрическая прогрессия

Answer 1

$S=c_{1} \frac{q^8-1}{q-1} .\\c_{4}= c_{1}q^3; c_{1}= \frac{c^4}{q^3} ; S= \frac{c^4}{q^3}\frac{q^8-1}{q-1} =\frac{1000}{125} \frac{5^8-1}{4} =781248$