Question

(20 символов)...........

Answer 1

$sn = \frac{b1 (1 - {q}^{n}) }{1 - q}$

$bn = b1 \times {q}^{n - 1} \\ b2 = b1 \times {q} \\ - 8 = - 4 \times q \\ q = 2$

$s7 = \frac{ - 4(1 - {2}^{7}) }{1 - 2} = - 508$

Ответ: S7=-508