Question

S4=909 g=10 найти b1

Answer 1

$S_4=909\\q=10\\b_1-?\\\\S_4= \frac{b_1(1-q^4)}{1-q}\; \; \; \; =\ \textgreater \ b_1= \frac{S_4(1-q)}{1-q^4}\\\\b_1= \frac{909(1-10)}{1-10^4}= \frac{909*(-9)}{1-10000}= \frac{-101*9*9}{-9999}= \frac{9}{11}$